How do Trinitarians explain the Athanasian Creed saying that the "one God" is the Trinity rather than the Father as in the Nicene Creed? In Scripture and the early Creeds, &quot;one God&quot; is used primarily to refer to God the Father:

1 Cor 8:6 (NIV): yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live.
Niceno-Constantinopolitan Creed of 381: We believe in one God, the Father Almighty, Maker of heaven and earth, and of all things visible and invisible.

But the Athanasian Creed differs by using the phrase &quot;one God&quot; to refer to the whole of the godhead or Trinity:

And the catholic faith is this: that we worship one God in Trinity, and Trinity in Unity; ... The Father is God, the Son is God and the Holy Spirit is God. They are not three Gods but one God.

What explains this shift from thinking first of the Father as God and then to the Son and Spirit as co-equal with the Father, to describing the Trinity as the first and main way we should understand &quot;one God&quot;?
Divinity &amp; unity appropriated to God the Father
As is evident in many of the Trinitarian verses listed below, the names &quot;God the Father&quot; and &quot;God&quot; simply are often used synonymously. This is because the Father is the principle of the divinity of the other Divine Persons, Who proceed from Him; He communicates to them the one divinity.
Holy Scripture and the Fathers appropriate certain attributes to each Person. Appropriation is

A manner of speaking in which the properties and activities of God, though common to the three divine persons, are attributed to an individual person. The purpose of appropriation is to manifest the differences in the divine properties and persons. Four kinds of appropriations are known from Scripture and sacred tradition:

substantive names of God (Theos), applied to the Father, and of Lord (Kurios), applied to the Son;

absolute attributes of God, namely power, unity, and eternity applied to the Father; wisdom, equality, and beauty applied to the Son; goodness, harmony, and happiness applied to the Holy Spirit;

works of God, namely efficient cause (Father), exemplary cause (Son), and final cause (Holy Spirit);

worship of God, with the Father as recipient of adoration and sacrifice, and the Son and Holy Spirit as mediators between God and man.


(Etym. Latin appropriatio, ascribing, the attributing of a special characteristic.)

See also Whether the essential attributes are appropriated to the persons in a fitting manner by the holy doctors?, where St. Augustine is quoted as saying (De Doctr. Christ. i, 5):

In the Father is unity, in the Son equality, in the Holy Spirit the harmony of unity and equality; and these three attributes are all one because of the Father, all equal because of the Son, and all harmonious because of the Holy Spirit.

Trinitarian verses
The New Testament not only mentions the Divine Persons of the Trinity

individually, but also frequently, more than forty times, they are mentioned
together or at the same time. The main Trinitarian texts are:

Gospels:


Matt. 3:16-17 (Mark 1:10-11; Luke 3:22), the theophany at the baptism of Christ.And Jesus being baptized, forthwith came out of the water: and lo, the heavens were opened to him: and he saw the Spirit of God descending as a dove, and coming upon him. And behold a voice from heaven [God the Father], saying: This is my beloved Son, in whom I am well pleased.

Matt. 28:19, the command to baptize. Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.

Luke 1:35, the annunciation of the Incarnation.And the angel answering, said to her: The Holy Ghost shall come upon thee, and the power of the most High [God the Father] shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God.

John 14:16, 17, 23; 15:26; 16:7-11, 12-15, the promise of the Holy Spirit at the Last Supper.And I [God the Son] will ask the Father, and he shall give you another Paraclete [Holy Ghost], that he may abide with you for ever. The spirit of truth [Holy Ghost], whom the world cannot receive, because it seeth him not, nor knoweth him: but you shall know him; because he shall abide with you, and shall be in you. […] Jesus answered, and said to him: If any one love me, he will keep my word, and my Father will love him, and we [with the Holy Ghost] will come to him, and will make our abode with him.[…]But when the Paraclete [Holy Ghost] cometh, whom I [God the Son] will send you from the Father, the Spirit of truth [Holy Ghost], who proceedeth from the Father, he [Holy Ghost] shall give testimony of me.[…]But I [God the Son] tell you the truth: it is expedient to you that I go: for if I go not, the Paraclete [Holy Ghost] will not come to you; but if I go, I will send him to you. And when he is come, he will convict the world of sin, and of justice, and of judgment. Of sin: because they believed not in me. And of justice: because I go to the Father; and you shall see me no longer. And of judgment: because the prince of this world is already judged. I have yet many things to say to you: but you cannot bear them now. But when he, the Spirit of truth [Holy Ghost], is come, he will teach you all truth. For he shall not speak of himself; but what things soever he shall hear, he shall speak; and the things that are to come, he shall shew you. He shall glorify me; because he shall receive of mine, and shall shew it to you. All things whatsoever the Father hath, are mine. Therefore I said, that he [Holy Ghost] shall receive of mine, and shew it to you.



St. Peter:


Acts 2:33, 38-39; 5:31-32, Peter’s first sermons.Being exalted therefore by the right hand of God, and having received of the Father the promise of the Holy Ghost, he [God the Son] hath poured forth this which you see and hear. […] But Peter said to them: Do penance, and be baptized every one of you in the name of Jesus Christ, for the remission of your sins: and you shall receive the gift of the Holy Ghost. For the promise is to you, and to your children, and to all that are far off, whomsoever the Lord our God [the Father] shall call.[…]Him [God the Son] hath God exalted with his right hand, to be Prince and Saviour, to give repentance to Israel, and remission of sins. And we are witnesses of these things and the Holy Ghost, whom God hath given to all that obey him.


St. Paul:


Rom. 8:9-11, 14-17; 1 Cor. 2:10-16; Eph. 1:17; 2:17-22, the divine action in the heart of the just.But you are not in the flesh, but in the spirit, if so be that the [Holy] Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. And if Christ be in you, the body indeed is dead, because of sin; but the spirit liveth, because of justification. And if the Spirit of him that raised up Jesus from the dead, dwell in you; he that raised up Jesus Christ from the dead, shall quicken also your mortal bodies, because of his [Holy] Spirit that dwelleth in you. […] For whosoever are led by the [Holy] Spirit of God, they are the sons of God. For you have not received the spirit of bondage again in fear; but you have received the spirit of adoption of sons, whereby we cry: Abba (Father). For the [Holy] Spirit himself giveth testimony to our spirit, that we are the sons of God. And if sons, heirs also; heirs indeed of God, and joint heirs with Christ: yet so, if we suffer with him, that we may be also glorified with him.[…]But to us God hath revealed them, by this [Holy] Spirit. For the [Holy] Spirit searcheth all things, yea, the deep things of God. For what man knoweth the things of a man, but the spirit of a man that is in him? So the things also that are of God no man knoweth, but the [Holy] Spirit of God. Now we have received not the spirit of this world, but the [Holy] Spirit that is of God; that we may know the things that are given us from God. Which things also we speak, not in the learned words of human wisdom; but in the doctrine of the [Holy] Spirit, comparing spiritual things with spiritual. But the sensual man perceiveth not these things that are of the [Holy] Spirit of God; for it is foolishness to him, and he cannot understand, because it is spiritually examined. But the spiritual man judgeth all things; and he himself is judged of no man. For who hath known the mind of the Lord, that we may instruct him? But we have the mind of Christ.[…]That the God of our Lord Jesus Christ, the Father of glory, may give unto you the spirit of wisdom and of revelation, in the knowledge of him:[…]And coming, he [Jesus] preached peace to you that were afar off, and peace to them that were nigh. For by him we have access both in one [Holy] Spirit to the Father. Now therefore you are no more strangers and foreigners; but you are fellow citizens with the saints, and the domestics of God, Built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone: In whom all the building, being framed together, groweth up into an holy temple in the Lord. In whom you also are built together into an habitation of God in the [Holy] Spirit.

1 Cor. 6:15-20, Christians as the temple of God.Know you not that your bodies are the members of Christ? Shall I then take the members of Christ, and make them the members of an harlot? God forbid. Or know you not, that he who is joined to a harlot, is made one body? For they shall be, saith he, two in one flesh. But he who is joined to the Lord, is one spirit. Fly fornication. Every sin that a man doth, is without the body; but he that committeth fornication, sinneth against his own body. Or know you not, that your members are the temple of the Holy Ghost, who is in you, whom you have from God; and you are not your own? For you are bought with a great price. Glorify and bear God in your body.

1 Cor. 12:3-6, the distribution of charisms.Wherefore I give you to understand, that no man, speaking by the [Holy] Spirit of God, saith Anathema to Jesus. And no man can say the Lord Jesus, but by the Holy Ghost. Now there are diversities of graces, but the same [Holy] Spirit; And there are diversities of ministries, but the same Lord; And there are diversities of operations, but the same God, who worketh all in all.

2 Cor. 13:13, benediction and greeting.The grace of our Lord Jesus Christ, and the charity of God [the Father], and the communication of the Holy Ghost be with you all. Amen.

Gal. 4:4-9,14, the mission of Christ.But when the fulness of the time was come, God sent his Son, made of a woman, made under the law: That he might redeem them who were under the law: that we might receive the adoption of sons. And because you are sons, God hath sent the [Holy] Spirit of his Son into your hearts, crying: Abba, Father. Therefore now he is not a servant, but a son. And if a son, an heir also through God. But then indeed, not knowing God, you served them, who, by nature, are not gods. But now, after that you have known God, or rather are known by God: how turn you again to the weak and needy elements, which you desire to serve again? […] You despised not, nor rejected: but received me as an angel of God, even as Christ Jesus. 

Eph. 3:14-19, the prayer of Paul.For this cause I bow my knees to the Father of our Lord Jesus Christ, Of whom all paternity in heaven and earth is named, That he would grant you, according to the riches of his glory, to be strengthened by his [Holy] Spirit with might unto the inward man,That Christ may dwell by faith in your hearts; that being rooted and founded in charity, You may be able to comprehend, with all the saints, what is the breadth, and length, and height, and depth: To know also the charity of Christ, which surpasseth all knowledge, that you may be filled unto all the fulness of God.

Eph. 4:4-6, the unity of the Church.One body and one [Holy] Spirit; as you are called in one hope of your calling. One Lord, one faith, one baptism. One God and Father of all, who is above all, and through all, and in us all.

Tit. 3:4-6, our regeneration.But when the goodness and kindness of God our Saviour [Jesus] appeared: Not by the works of justice, which we have done, but according to his mercy, he saved us, by the laver of regeneration, and renovation of the Holy Ghost; Whom he hath poured forth upon us abundantly, through Jesus Christ our Saviour:

Hebr. 9:14, the sacrifice of Christ.How much more shall the blood of Christ, who by the Holy Ghost offered himself unspotted unto God, cleanse our conscience from dead works, to serve the living God?

Hebr. 10:29-31, the evil of apostasy.How much more, do you think he deserveth worse punishments, who hath trodden under foot the Son of God, and hath esteemed the blood of the testament unclean, by which he was sanctified, and hath offered an affront to the [Holy] Spirit of grace? For we know him that hath said: Vengeance belongeth to me, and I will repay. And again: The Lord shall judge his people. It is a fearful thing to fall into the hands of the living God.



St. Peter:


1 Pet. 1:1-2, greeting and benediction.Peter, an apostle of Jesus Christ, to the strangers dispersed through Pontus, Galatia, Cappadocia, Asia, and Bithynia, elect, According to the foreknowledge of God the Father, unto the sanctification of the [Holy] Spirit, unto obedience and sprinkling of the blood of Jesus Christ: Grace unto you and peace be multiplied. 

1 Pet. 4:14, reproach in the name of Christ.If you be reproached for the name of Christ, you shall be blessed: for that which is of the honour, glory, and power of God, and that which is his [Holy] Spirit, resteth upon you.



St. John:


1 John 4:11-16, charity.My dearest, if God hath so loved us; we also ought to love one another. No man hath seen God at any time. If we love one another, God abideth in us, and his charity is perfected in us. In this we know that we abide in him, and he in us: because he hath given us of his [Holy] spirit. And we have seen, and do testify, that the Father hath sent his Son to be the Saviour of the world. Whosoever shall confess that Jesus is the Son of God, God abideth in him, and he in God. And we have known, and have believed the charity, which God hath to us. God is charity: and he that abideth in charity, abideth in God, and God in him.


source: Sacræ Theologiæ Summa IIA: One &amp; Triune God p. 250