What is the biblical basis for the Athanasian Creed's statement that believing in the Trinity is necessary for salvation? The Athanasian Creed states:


  Whosoever will be saved, before all things it is necessary that he hold the catholic faith. Which faith except every one do keep whole and undefiled; without doubt he shall perish everlastingly. And the catholic faith is this: That we worship one God in Trinity, and Trinity in Unity; Neither confounding the Persons; nor dividing the Essence. . . . He therefore that will be saved, let him thus think of the Trinity.
  
  Furthermore, it is necessary to everlasting salvation; that he also believe faithfully the Incarnation of our Lord Jesus Christ. . . . This is the catholic faith; which except a man believe truly and firmly, he cannot be saved.


I think the biblical basis is easy to establish for the idea that believing the incarnation is necessary for salvation, but what is the biblical basis for the idea that believing in the Trinity is necessary for salvation?
Jesus himself equates salvation with knowing God:

Now this is eternal life: that they know you, the only true God, and Jesus Christ, whom you have sent. - John 17:3 NIV

Elsewhere, the scriptures negate the idea that this could be a mere intellectual knowledge about God:

You believe that there is one God. Good! Even the demons believe that—and shudder. - James 2:19 NIV (cf. in context)

Instead, examining the preceding context (cf. John 14, 15 &amp; 16) we must understand this knowing of God in terms of a relationship that includes intimate familiarity with His nature, which is revealed in the persons of Father, Son and Holy Spirit and consequently includes &quot;believing in the Trinity&quot;:

7 If you really know me, you will know my Father as well. From now on, you do know him and have seen him.”
8 Philip said, “Lord, show us the Father and that will be enough for us.”
9 Jesus answered: “Don’t you know me, Philip, even after I have been among you such a long time? Anyone who has seen me has seen the Father. How can you say, ‘Show us the Father’?  - John 14:7-9 NIV
15 “If you love me, keep my commands. 16 And I will ask the Father, and he will give you another advocate to help you and be with you forever— 17 the Spirit of truth. The world cannot accept him, because it neither sees him nor knows him. But you know him, for he lives with you and will be in you. 18 I will not leave you as orphans; I will come to you. 19 Before long, the world will not see me anymore, but you will see me. Because I live, you also will live. 20 On that day you will realize that I am in my Father, and you are in me, and I am in you. - John 14:15-20 NIV
I no longer call you servants, because a servant does not know his master’s business. Instead, I have called you friends, for everything that I learned from my Father I have made known to you. John 15:15 NIV
When the Advocate comes, whom I will send to you from the Father—the Spirit of truth who goes out from the Father—he will testify about me. - John 15:26 NIV
13 But when he, the Spirit of truth, comes, he will guide you into all the truth. He will not speak on his own; he will speak only what he hears, and he will tell you what is yet to come. 14 He will glorify me because it is from me that he will receive what he will make known to you. 15 All that belongs to the Father is mine. That is why I said the Spirit will receive from me what he will make known to you. - John 16:13-15 NIV
29 Then Jesus’ disciples said, “Now you are speaking clearly and without figures of speech. 30 Now we can see that you know all things and that you do not even need to have anyone ask you questions. This makes us believe that you came from God.” - John 16:29-30 NIV

In summary, in much of his extended discourse following the last supper, Jesus teaches his disciples regarding the persons of the Trinity with the expressed intent that they may know God; and in the midst of his &quot;High-priestly prayer&quot; in chapter 17, he equates knowing this God (ie the Trinity) with eternal life.