St Thomas mistrusted the promise of Jesus and refused to believe that He had risen. Our Lord showed Him great mercy and forbearance by giving Him the chance to repent. When he fell down on his knees and humbly professed His faith in Jesus “My Lord and my God”, Our Lord immediately forgave him. The apostles would have been familiar with all of the texts of the Old Testament proclaiming the steadfast mercy of God and would have rejoiced to see this shown in the living and risen Messiah.

St Alphonsus explains that the mercy of God is infinite, far beyond the mercy of even the greatest saint. He says:

Oh, if we could but understand the love that burns in the Heart of Jesus for us! He has loved us so much, that if all men, all the Angels, and all the Saints were to unite with all their energies, they could not arrive at the thousandth part of the love that Jesus bears to us. He loves us infinitely more than we love ourselves.

God seeks our good both here and for all eternity, and He longs to give us His gifts of grace far more than we desire to receive them. We might think that we are loving ourselves or doing ourselves good by our moral and spiritual weakness, but the Lord created us and loves us with an infinite love. He knows that the life of grace brings us far greater benefits than any earthly goods.

Even so, God will not force us to love Him because love can only be truly such if it is freely given. God created us with a spiritual soul precisely so that we could actually love freely and without compulsion. We are also able to reject God’s mercy by sinning.
If we insult God by sin, we must not insult Him further by abusing His mercy and continuing in the same sin. As St Alphonsus says again:

The sinner says: But God is merciful. I reply: Who denies it? The mercy of God is infinite; but with all that mercy, how many are lost every day! I come to heal the contrite of heart. (Is 61.1). God heals those who have a good will. He pardons sin; but He cannot pardon the determination to sin.

Or as he put it in another place:

Our Lord exercises mercy toward those who fear offending Him, but not toward those who use His mercy as a pretext to insult Him.

The Saint is referring here to the very words of Our Lady herself in the Magnificat “His mercy is from generation to generation upon those who fear Him.” (Lk 1.50) This fear is not a servile fear as we might have for a tyrant, it is the noble fear of offending the one who is infinitely good and benevolent towards us.

St Alphonsus gave an easy rule for putting into practice both the fear of God and trust in His mercy. He said that after we have sinned, we should not despair, we should hope for His mercy; but before sin, when we are tempted to sin, we should fear His divine justice. We might do well also to remember at those times of trial the words of Our Lady: “His mercy is upon those who fear Him.”