From the Office of Readings:
"He saw the tax collector and, because he saw him through the eyes of mercy and chose him, he said to him: Follow me. This following meant imitating the pattern of his life --not just walking after him. St. John tells us: Whoever says he abides in Christ ought to walk in the same way in which he walked.
And he rose and followed him. There is no reason for surprise that the tax collector abandoned earthly wealth as soon as the Lord commanded him. Nor should one be amazed that neglecting his wealth, he joined a band of men whose leader had, on Matthew's assessment, no riches at all. Our Lord summoned Matthew by speaking to him in words. By an invisible, interior impulse flooding his mind with the light of grace, he instructed him to walk in his footsteps. In this way Matthew could understand that Christ, who was summoning him away from earthly possessions, had incorruptible treasures of heaven in his gift.
As he sat at table in the house, behold many tax collectors and sinners came and sat down with Jesus and his disciples. This conversion of one tax collector gave many men, those from his own profession and other sinners, an example of repentance and pardon. Notice also the happy and true anticipation of his future status as apostle and teacher of the nations. No sooner was he converted than Matthew drew after him a whole crowd of sinners along the same road to salvation. He took up his appointed duties while still taking his first steps in the faith, and from that hour he fulfilled his obligation and thus grew in merit. To see a deeper understanding of the great celebration Matthew held at his house, we must realise that he not only gave a banquet for the Lord at his earthly residence, but far more pleasing was the banquet set in his own heart which he provided through faith and love. Our Saviour attests to this: Behold I stand at the door; if anyone hears my voice and opens the door, I will come in to him and eat with him, and he with me.
Let me recommend a great commentary on Matthew's Gospel. It is written by a non-Catholic but is magisterial with a few exceptions and will definitely leave the reader satiated with knowing a lot more than they did before. It is in two volumes:
and Volume II: