Why are we able to truly believe the gospel?
We are able to truly believe the gospel because God has literally opened up our ability to believe these things. True belief is heart belief, not merely mental assent or head knowledge, and God enables us to trust our whole eternal destiny on it.
Les teaches that believing the gospel means trusting, having faith in it, and it must be a true heart belief rather than just a head knowledge. He notes that many people miss heaven by twelve inches—they had it in the head but never got it in the heart. The only reason anyone can believe is because God has literally opened up their ability to believe and trust their whole eternal destiny on it. Without that kind of heart faith, there is an eternal doom coming for the lost, which is awful and cannot be compromised. For those who believe, it begins in the heart.