At that time Jesus came to Jericho and intended to pass through the town. Now a man there named Zacchaeus, who was a chief tax collector and also a wealthy man, was seeking to see who Jesus was; but he could not see him because of the crowd, for he was short in stature. So he ran ahead and climbed a sycamore tree in order to see Jesus, who was about to pass that way. When he reached the place, Jesus looked up and said, “Zacchaeus, come down quickly, for today I must stay at your house.” And he came down quickly and received him with joy. When they saw this, they began to grumble, saying, “He has gone to stay at the house of a sinner.” But Zacchaeus stood there and said to the Lord, “Behold, half of my possessions, Lord, I shall give to the poor, and if I have extorted anything from anyone I shall repay it four times over.” And Jesus said to him, “Today salvation has come to this house because this man too is a descendant of Abraham. For the Son of Man has come to seek and to save what was lost.”
Observe the gracious kindness of the Savior. The innocent associates with the guilty, the fountain of justice with greed, the source of injustice. Having entered the publican's house, Jesus is not stained from the mists of avarice, but disperses them by the bright beam of His righteousness. But those who deal with biting words and reproaches, try to cast a slur upon the things which were done by Him; for it follows, "And when they saw it, they all murmured, saying, That he was gone to be guest with a man that is a sinner."
But Jesus, though accused of being a wine-drinker and a friend of publicans, ignored them, so long as He could accomplish His end. As a physician sometimes cannot save his patients from their diseases without exposure to blood. So it happened here, for the publican was converted, and lived a better life. Zacchaeus stood, and said to the Lord, "Behold, Lord, the half of my goods I give to the poor; and if I have defrauded any man, I restore him fourfold." Here is a miracle: without learning he obeys. Zacchaeus did not wait for the judgment of the law, but makes himself his own judge, demonstrating a love for God by hating his sins. And as the sun pouring its rays into a house enlightens it not by word, but by work, so the Savior by the rays of righteousness put to flight the darkness of sin; for the light shines in darkness.
Catena Aurea (St. Thomas Aquinas)