God is Merciful, But He is Also Just
St. Augustine writes: "Woe to him who hopes in order to sin! How many have gone to their perdition relying on the shadow of vain hope!"
"Unhappy the man who abuses God's mercy to offend Him!" says St. Bernard. In short, God is patient, but a great number of adults, even among Christians, are damned. "It is a broad gate and a wide road that leads to perdition, and those that go that way are many indeed" (Matt. 7:13).
According to St. Augustine, he who offends God with the hope of being forgiven "is a scoffer, not a penitent." But St. Paul writes, "You cannot cheat God" (Gal. 6:7). And it would be indeed cheating God if we offended God whenever it pleased us, in order to repent later on and go to heaven. He who sows sin will reap only punishment and hell, because "a man will reap what he sows" (Gal. 6:8).
The net with which Satan drags sinners to hell is the vain hope that he gives them, saying, "Sin freely, because even with all your sins you will be saved." But God curses the sinners who sin with the hope of mercy, for "no rescue shall ever reach them; no other prospect have they but despair" (Job 11:20). Such vain hope provokes God's anger, just as in the case of a servant who, taking advantage of his master's goodness, insults him.
From St. Alphonsus Liguori's, Preparation for Death
(Consideration XVII, 2)