What happens to a person who sins after his baptism?

This question was presented to St. Basil the Great, due to the spread of a certain idea that whoever falls into grave sins after baptism has no hope. The saint affirmed that we should never lose hope as long as our repentance is sincere and bears the fruit of our return to God, who awaits all. St. Augustine says that as long as we have breath, God waits for our repentance, or else why did He not take the soul away?

Will he fall into despair of his own salvation, he who fell in a multitude of sins after his baptism? What is the measure by which someone can continue to be hopeful in God’s love through repentance?

St. Basil the Great responds: “If we compare God’s ultimate compassion or abundant mercy, to the number of sins and their heaviness, we will find no place for despair. Even if the latter can be measured or counted, it is impossible to measure God’s compassion or to count His mercies, so despair can by no means find place. By the knowledge of mercy and condemnation of sin, we find forgiveness in the blood of Christ as it is written in Matthew 26:28. There are many methods and paths by which we learn that we ought not to despair. In particular, we look at the parable by our Lord Jesus Christ, about the son who wasted his fathers’ wealth on sin. From words of the Lord Himself, we learn what kind of great banquet it was, that which befits repentance (Luke 15:22–24). I add to that what God said through Isaiah: “Though your sins are like scarlet, they shall be as white as snow; though they are red like crimson, they shall be as wool” (Isa. 1:18). We need to be aware though, that this is only achieved when our repentance is truly worthy of this, and if it is the result of our hatred to sin, as it is written in the Old and New Testaments, bearing fruits worthy of it.”¹⁴⁵⁴