Saint Augustine on the reason WHY Faith is given only to some and not to all.

Faith, then, as well in its beginning as in its completion, is God's gift; and let no one have any doubt whatever, unless he desires to resist the plainest sacred writings, that this gift is given to some, while to some it is not given. But why it is not given to all ought not to disturb the believer, who believes that from one all have gone into a condemnation, which undoubtedly is most righteous; so that even if none were delivered therefrom, there would be no just cause for finding fault with God. Whence it is plain that it is a great grace for many to be delivered, and to acknowledge in those that are not delivered what would be due to themselves; so that he that glories may glory not in his own merits, which he sees to be equalled in those that are condemned, but in the Lord. But why He delivers one rather than another,— "His judgments are unsearchable, and His ways past finding out" (Romans 11:33) For it is better in this case for us to hear or to say, "O man, who are you that repliest against God?" (Romans 9:20) than to dare to speak as if we could know what He has chosen to be kept secret. Since, moreover, He could not will anything unrighteous. [Saint Augustine, On the Predestination of the Saints, Book I, Chapter 16]