When God's mercies have reached their end He punishes, and pardons no more. God is merciful; but, as great as His mercy is, how many people He sends to Hell every day! God is merciful but He is also just; and He is therefore obliged to punish those who offend Him. When sins reach a certain number, God pardons no more. St. Basil, St. Cyril of Alexandria, St. Jerome, St. Ambrose, St. Augustine, St. John Chrysostom, and other Fathers teach that, according to the words of Scripture: "Thou hast ordered all things in measure and number and weight" (Wisdom 11: 2 1), God has fixed for each person the number of sins He will pardon; and when this number is completed, He will pardon no more. God does bear with us, but not forever. When the time comes for vengeance, He punishes. How many God has sent to Hell for the first offense! St. Gregory relates that a child five years old was seized by the devil for having uttered a blasphemy and carried into Hell. Another of eight, after his first sin, died and was lost. St. Alphonsus Maria Liguori
How many souls have been damned for a single mortal sin! St. Ignatius of Loyola
Faith is a gift from God. And let no one have any doubt whatsoever that, while this gift is given to some, to others it is not given. Why it is not given to everyone ought not disturb the faithful; even if no one were delivered, there would be no just cause for finding fault with God! St. Augustine
Neither faith nor works shall free from eternal punishment any of those who die in mortal sin, even after any length of time whatever. It would seem that by the mercy of God all punishment of the damned comes to an end. Since, however, His mercy is ruled by His wisdom, it does not reach certain people who render themselves unworthy. And yet, the mercy of God is found even in them, in that they are punished less than they deserve. St. Thomas Aquinas