Hey Blaine,
My favorite "proof text" for the Trinity, against the modalism heresy, comes from Jesus conversation with the Pharisees in John 8:12-20.
12 Then Jesus again spoke to them, saying, "I am the Light of the world; he who follows Me will not walk in the darkness, but will have the Light of life." 13 So the Pharisees said to Him, "You are testifying about Yourself; Your testimony is not true." 14 Jesus answered and said to them, "Even if I testify about Myself, My testimony is true, for I know where I came from and where I am going; but you do not know where I come from or where I am going. 15You judge according to the flesh; I am not judging anyone. 16 But even if I do judge, My judgment is true; for I am not alone in it, but I and the Father who sent Me. 17 Even in your law it has been written that the testimony of two men is true. 18 I am He who testifies about Myself, and the Father who sent Me testifies about Me." 19So they were saying to Him, "Where is Your Father?" Jesus answered, "You know neither Me nor My Father; if you knew Me, you would know My Father also." 20These words He spoke in the treasury, as He taught in the temple; and no one seized Him, because His hour had not yet come.
In Jesus conversation with the Pharisees, they tell Jesus that his testimony is false, because he is testifying about himself. Jesus reply to them is telling. First, he says in verse 14 that his testimony is true because he knows where he came from, but even if his personal experience wasn't enough for them, he says in v 18 "My judgments is true, for I am not alone in it." Then he appeals to the Law of Moses which says "that the testimony of two is true." (v 17) Then he says in v 18 "I am He who testifies about Myself, and the Father who sent Me testifies about Me."
It is evident to me from this text that to prove the authenticity of his testimony, Jesus appeals to the law which says the testimony of "two" is true. Two what? Two people. Therefore, Jesus says His testimony is true because He testifies about Himself, and He is "not alone" in this testimony, because the Father also testifies on behalf of Christ.
This account is enough, in my mind, to show that the Father and Son are two distinct persons, co-existing in time separately from one another. If Jesus and the Father were the exact same person, and not co-existing, and God sometimes wears the mask of Father and sometimes wears the mask of Son, then Jesus could not appeal to two distinct testimonies as demanded by the law of Moses in order to win his argument with the Pharisees.
While I am sypmathetic to people who hold modalistic views of the Trinity outrightly, having personally done so in my younger Christian days, I believe those who hold such an opinion are in gross error, and whose viewpoint will ultimately harm if not outright destroy their relationship with God.
I would be careful of pronouncing everybody who holds such a view as damned, but as one theologian I once read said something to this effect:
Our knowledge and understanding of the Trinity is not merely abstract head knowledge, but is a real relationship issue. If God is in fact a trinity, how can somebody be in a relationship with each member of the godhead and not know it?
I would say I subscribe to such a thought. While there is much room for grace and for growth, if a person holds a prolonged and dogmatic modalistic view, I would say their relationship with the Lord has long been fractured.