St Thomas seems to have been devoted to St Nicholas, who was a very popular saint in the Middle Ages. The Dominican Church of St Dominic in Naples has a chapel dedicated to St Nicholas. Thomas would offer his morning Mass in that chapel and pray there for long hours.
It was in that chapel on the feast of St Nicholas in 1273 that Thomas had a mysterious mystical experience that changed his life. Something happened while he was offering Mass, as if he was caught up in an ecstatic state. Afterward he was profoundly changed. Up to then Thomas had spent many, many hours writing. But after that day, this great Doctor of the Church put down his pen and stopped writing.
He had not finished his crowning achievement, the Summa Theologiae. His secretary Friar Reginald begged him to keep on writing. But Thomas simply replied, "Reginald, I cannot. All I have written seems to me as straw in comparison with what I have seen." Three months later, on March 7, 1274, St Thomas died.
A few years earlier, when Thomas was still in Paris, he had preached a beautiful homily on St Nicholas, in which he emphasizes mercy. As a pastor, the works of the saint especially focused on mercy. The stories that have been passed on about him show this, as for example the time that Nicholas secretly provided the dowries for three young women. Some quotes:
"The principal work of the Lord is mercy, as the Psalmist says, 'His tender mercies are over all his works' (Ps 144:9). The Lord's servant is one who exercises mercy toward the poor."
"We use oil to heal a wound, through which we understand healing grace... And since blessed Nicholas was anointed with the oil of healing grace, because he had full soundness of spiritual health and was equipped to anoint others, we are told that wine and oil were poured--that is, the wine of stern correction and the oil of mercy and comfort."
"We use oil to soften, and this signifies mercy and kindness of heart, both of which blessed Nicholas possessed, since he was utterly filled with mercy and devotion.. . . Just as oil spreads over things, mercy spreads over every good work. Unless you have mercy, your labors are nothing."