Is the Pope the Antichrist or the spirit of the Antichrist? According to reformed theology, in the Savoy Declaration of Faith, Chapter 26 (Of the Church), paragraph 4, the Pope is the Antichrist.

There is no other Head of the Church but the Lord Jesus Christ; nor can the Pope of Rome in any sense be head thereof; but is that antichrist, that man of sin, and son of perdition, that exalteth himself in the Church against Christ, and all that is called God, whom the Lord shall destroy with the brightness of his coming.

Also in the 1689 Baptist Confession of Faith, Chapter 26 (Of the Church), paragraph 4, we found the same, the Pope as the Antichrist.

The Lord Jesus Christ is the Head of the church, in whom, by the appointment of the Father, all power for the calling, institution, order or government of the church, is invested in a supreme and sovereign manner; neither can the Pope of Rome in any sense be head thereof, but is that antichrist, that man of sin, and son of perdition, that exalts himself in the church against Christ, and all that is called God; whom the Lord shall destroy with the brightness of his coming.

But how can he be THE Antichrist, as 2 Thessalonians 2: 2-9 says, if it is talking about a position and not a person. Would not be more in line with the concept of the spirit of the antichrist, of which it is spoken of in 1 John 2: 18-19 and 1 John 4: 2-3.
How can this paragraph be interpreted? The Pope is an antichrist or the Pope is the Antichrist?
If I say 'B is that A' it does not exclude the possibility of also saying 'C is that A'.

That the Savoy Declaration (and others that followed it) states the Pope of Rome as being 'Antichrist' does not mean that the authors meant to exclusively define 'Antichrist' to mean 'Pope of Rome'. They were locating what, to them (as Reformers), was the prime example of what they considered to be 'antichrist' in their own experience and in their own times.
The wording is quite precise :

... is that antichrist.

The very definition of the word 'antichrist' (according to the meaning of the Greek prefix 'anti' see Strong 473) is that it is in opposition to  Christ.
It is any form of religion that, rising up in an organised manner, globally, competes with Christ, as the King of Kings, reigning (by divine and angelic power) from the throne of God in heaven and competes with Christ, as the Head of the Church, reigning (in Spirit) over the Body of Christ gathered on earth.
To say, in English, 'that' antichrist does not mean it is 'the' antichrist.
The demonstrative pronoun 'that' does not have the power of the definite article 'the', in English, or in other languages. It locates something as being an example of a concept. It does not define that which is located as being the only example of the concept.
I may point to my pet, if I am displeased with its lack of loyalty to me, and say 'that Judas' meaning it has betrayed me in some fashion. But I am by no means asserting that my pet is the only representation on earth, presently, which bears treacherous characteristics.
I believe the wording of the Savoy Declaration (and its followers) is quite precise and demonstrates your alternative in your question 'an antichrist'.