In Christianity, one of the most common reasons Christians appeal to to justify their belief in Christianity is the so-called "inner witness of the Holy Spirit". William Lane Craig, a renowned Christian apologist, puts it this way (emphasis mine):
[...] These bothers and sisters endured horrible oppression and atheistic indoctrination by the Marxist regime and yet did not abandon Christ. As I emphasized in my answer to Question #13, evidence varies from generation to generation and from place to place and is accessible only to those privileged few who have the education, leisure time, and resources to explore it. God has provided a more secure basis for our faith than the shifting sands of evidence and argument, namely, the indwelling Holy Spirit.
Moreover, this conclusion seems in line with New Testament teaching on the witness of the Holy Spirit. While non-believers reject New Testament teaching, Christians should take it seriously. Ponder, then, John's words:
And the Spirit is the witness, because the Spirit is the truth. . . . If we receive the testimony of men, the testimony of God is greater; for this is the testimony of God that he has borne witness to his Son. He who believes in the Son of God has the testimony in himself. He who does not believe God has made him a liar, because he has not believed in the testimony that God has borne to his Son (1 John 5:6-10).
As Christian believers we have the testimony of God living within us, the Holy Spirit, whose testimony exceeds in force all human testimony.
So in answer to your question, Kyle, I think that in fact God will not allow someone to be in a position in which the rational thing for him to do is to reject God and Christ and separate himself from God. Given that God is essentially all-loving, I'm inclined to say that such a thing will not only never happen, but that it is, indeed, impossible. It follows that Christians who have apostatized have done so in defiance of the Holy Spirit's work by quenching or grieving the Spirit, so that what they did was in the end irrational.
Does that imply, Adam, as your sceptic says, that I think "evidence is unimportant when compared with faith?" No, because he's drawing a false contrast, comparing apples with oranges. Faith is not the issue here, but the ground for faith. Must the ground for faith be evidence? That is the question. We've already seen that evidentialism is bankrupt. Many of the things we know are not based on evidence. So why must belief in God be so based? Belief in God and the great truths of the Gospel is not a blind exercise of faith, a groundless leap in the dark. Rather, as Plantinga emphasizes, Christian belief is part of the deliverances of reason, grounded in the inner witness of the Holy Spirit, which is an objective reality mediated to me from God.
What is true is that evidence, as it is defined in these discussions, plays a secondary role compared to the role God Himself plays in warranting Christian belief. Should we, then, ignore strong evidence if it shows that our faith is probably false? Of course not! My work as a philosopher exemplifies the effort to confront objections to Christian belief squarely and to answer them. But most Christians in the world don't have that luxury. For them they may have to hold to their Christian belief even though they lack an answer to the alleged defeater. What I insist on is that, given the witness of the Holy Spirit within them, they are entirely rational in so doing.
Question: Is there an equivalent to the "inner witness of the Holy Spirit" in Islam? Does Allah provide a supernatural means for Muslims to reach 100% epistemological certainty that Islam is true, without having to be an expert in Islam apologetics, in a manner equivalent or similar to the "inner witness of the Holy Spirit" of Christianity?