1. There can only be either one or no true religion, for if two religions differ (teach contradictory things), they cannot be both true.
2. Either God has revealed Himself or not.
a) If He hasn't, all religions are man-made and thus false.
b) But if He has, there are two options: this religion is known or it isn't.
b.1) If it's not known, then we are, in practice, in the same situation as a).
b.2) But if it is, then there is exactly one true religion.
Since God is the Truth, He never lies, therefore He cannot have revealed anything which is false.
Since God is all-powerfull and provident, He must have established a way in which the true religion He revealed could get to our time unaltered and unblemished, teaching the same truths He revealed.
Thus, if we are in the case b.2), there is exactly one and only one true religion which has reached our time.
Since God is the Truth and never lies, and since He cannot love or desire evil or falsehood, all religions which are not revealed by God (that is, all false and man-made religions) are not willed by God; He does not desire them to exist. BUT He tolerates for some time their existence, as He tolerates other evils to get a greater good out of them (or to respect man's free will).
Now, some may say that God could have revealed more than one religion. In that case, He would have revealed just different "parts" (truths) of the same religion to different people. However, today's religions contradict each other, so it can't be the case that two nowadays religions are true (revealed by God).
Religion is not man seeking God, it's God who reveals Himself to man.
And He has made Himself known to us, He himself has spoken to us:
"The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the one and only Son, who came from the Father, full of grace and truth." John 1:14.
