The history of this conversion is fully given in the epistle taken from the Acts.
The Introit of the Mass is as follows:
“I know Whom I have believed, and I am certain that He is able to keep that which I have committed unto Him against that day, being a just judge. Lord, Thou hast proved me and known me, Thou hast known my sitting down, and my rising up.”
Glory be to the Father, and to the Son, and to the Holy Spirit. As it was in the beginning, is now, and ever shall be, world without end. Amen.
O God, Who didst teach the whole world by the preaching of blessed Paul the apostle, grant us, we beseech Thee, that we, who this day celebrate his conversion, may advance towards Thee by his example. Through Christ our Lord, etc.
Epistle: Acts 9:1-22
In those days: Saul as yet breathing out threatenings and slaughter against the disciples of the Lord, went to the high priest, and asked of him letters to Damascus, to the synagogues, that if he found any men and women of this way, he might bring them bound to Jerusalem. And as he went on his journey, it came to pass that he drew nigh to Damascus; and suddenly a light from heaven shined round about him. And falling on the ground, he heard a voice saying to him: Saul, Saul, why persecutest thou Me? Who said: Who art Thou, Lord? And He said: I am Jesus Whom thou persecutest. It is hard for thee to kick against the goad. And he trembling and astonished, said: Lord, what wilt Thou have me to do? And the Lord said to him: Arise, and go into the city, and there it shall be told thee what thou must do. Now the men who went in company with him stood amazed, hearing indeed a voice, but seeing no man. And Saul arose from the ground, and when his eyes were opened, he saw nothing. But they leading him by the hands, brought him to Damascus. And he was there three days without sight, and he did neither eat nor drink. Now there was a certain disciple at Damascus, named Ananias; and the Lord said to him in a vision: Ananias. And he said: Behold I am here, Lord. And the Lord said to him: Arise, and go into the street that is called Strait, and seek in the house of Judas, one named Saul of Tarsus. For behold he prayeth. (And he saw a man named Ananias, coming in and putting his hands upon him, that he might receive his sight.) But Ananias answered: Lord, I have heard by many of this man, how much evil he hath done to Thy saints in Jerusalem. And here he hath authority from the chief priests, to bind all that invoke Thy name. And the Lord said to him: Go thy way, for this man is to Me a vessel of election, to carry My name before the gentiles, and kings, and the children of Israel. For I will show him how great things he must suffer for My name’s sake. And Ananias went his way, and entered into the house, and laying his hands upon him, he said: Brother Saul, the Lord Jesus hath sent me, He that appeared to thee in the way as thou camest, that thou mayst receive thy sight, and be filled with the Holy Ghost. And immediately there fell from his eyes as it were scales, and he received his sight and rising up he was baptized. And when he had taken meat he was strengthened. And he was with the disciples that were at Damascus for some days. And immediately he preached Jesus in the synagogues, that He is the Son of God. And all that heard him were astonished, and said: Is not this he who persecuted in Jerusalem those that called upon this name; and came hither for that intent, that he might carry them bound to the chief priests? But Saul increased much more in strength, and confounded the Jews who dwelt at Damascus, affirming that this is the Christ.
What do we learn from this history?
Not to despise any sinner, nor to despair of his salvation; for, like Paul on the road to Damascus, the greatest sinner may, by the grace of God, be suddenly converted, and become a saint. At the command of God he accepted Ananias as his leader in the way of salvation, and became as zealous for the honor of Christ as he had previously been intent on persecuting Him. In like manner, a convert must shut his eyes to all by which he has heretofore been led astray, and must give heed to that only which God commands.
Gospel: Matthew 19:27-29
At that time Peter said to Jesus: Behold, we have left all things, and have followed Thee; what, therefore, shall we have? And Jesus said to them: Amen I say to you, that you, who have followed Me, in the regeneration, when the Son of man shall sit on the seat of His majesty, you also shall sit on twelve seats judging the twelve tribes of Israel. And every one that hath left house, or brethren, or sisters, or father, or mother, or wife, or children, or lands, for My name’s sake, shall receive an hundredfold, and shall possess life everlasting. This gospel teaches that he who renounces the world, its pleasures and its riches, shall receive the grace of God, virtues, interior consolation, and eternal happiness, which are a hundredfold, that is, infinitely, more precious than worldly goods.
O Saint Paul, great apostle, who, from being an enemy, became the most zealous friend and preacher of Christ, procure for me from Him, I beseech thee, grace at last truly to know Him, Whom I have heretofore so often denied, offended, and, by my sins, crucified anew; to follow Him, and, after thy examples to be henceforth as diligent in doing justice as I have formerly been in practising evil; that I may one day attain to that happiness which thou hast gained. Through Jesus Christ our Lord, etc. Amen.