Thomas er mest kjent som filosof og teolog, men han æres også, spesielt blant sine brødre og søstre i Ordenen, som mystiker og en læremester i bønn og det åndelige liv
I sin Summa Theologiae skriver han både om Guds barmhjertighet og om barmhjertighet som dyd for de troende:
God acts mercifully, not indeed by going against His justice, but by doing something more than justice; thus a man who pays another two hundred pieces of money, though owing him only one hundred, does nothing against justice, but acts liberally or mercifully. The case is the same with one who pardons an offence committed against him, for in remitting it he may be said to bestow a gift. Hence the Apostle calls remission a forgiving: "Forgive one another, as Christ has forgiven you" (Ephesians 4:32). Hence it is clear that mercy does not destroy justice, but in a sense is the fulness thereof. And thus it is said: "Mercy exalteth itself above judgment" (James 2:13).
(Summa Theologiae Ia, q. 21, a. 3, ad secundum)
A virtue may take precedence of others in two ways: first, in itself; secondly, in comparison with its subject. On itself, mercy takes precedence of other virtues, for it belongs to mercy to be bountiful to others, and, what is more, to succor others in their wants, which pertains chiefly to one who stands above. Hence mercy is accounted as being proper to God: and therein His omnipotence is declared to be chiefly manifested [Collect, Tenth Sunday after Pentecost].
(Summa Theologiae IIa IIae, q. 30, a. 4, respondeo)