That's a good question, and I think some of the details will of course play out differently from person to person. Yet, behind it there is a universal answer, I believe.
Romans 1:18-22 RSVA For the wrath of God is revealed from heaven against all ungodliness and wickedness of men who by their wickedness suppress the truth. (19) For what can be known about God is plain to them, because God has shown it to them. (20) Ever since the creation of the world his invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made. So they are without excuse; (21) for although they knew God they did not honor him as God or give thanks to him, but they became futile in their thinking and their senseless minds were darkened. (22) Claiming to be wise, they became fools,
John 3:17-21 RSVA For God sent the Son into the world, not to condemn the world, but that the world might be saved through him. (18) He who believes in him is not condemned; he who does not believe is condemned already, because he has not believed in the name of the only Son of God. (19) And this is the judgment, that the light has come into the world, and men loved darkness rather than light, because their deeds were evil. (20) For every one who does evil hates the light, and does not come to the light, lest his deeds should be exposed. (21) But he who does what is true comes to the light, that it may be clearly seen that his deeds have been wrought in God.
John 6:44-45 RSVA No one can come to me unless the Father who sent me draws him; and I will raise him up at the last day. (45) It is written in the prophets, 'And they shall all be taught by God.' Every one who has heard and learned from the Father comes to me.
I think those verses are pretty self-explanatory regarding my position, but I will elaborate if necessary.
To answer the other question, I could now choose to believe other than I do were I to be convinced that what I believe is false and the other true. Could I just up and say, "I'm gonna believe something new today."? No, clearly not. Belief is a matter that engages both the mind and the heart, and can neither be avoided nor taken lightly. Were it the case that I had been convinced, heart and mind, of the truth of a different belief when I was an atheist, then I suppose I could have taken on that belief instead. I think, for us to discuss this business of what one 'could' or 'could not' do or have done we need to define more clearly what we mean by 'could.' Of what sort of possibility are you speaking?
Thoughts?
T-A