The Apostle Paul, who went by the name of Saul before his conversion, had been persecuting Christians.  This is the record of his conversion.

Acts 9:1 And Saul, yet breathing out threatenings and slaughter against the disciples of the Lord, went unto the high priest,

2 And desired of him letters to Damascus to the synagogues, that if he found any of this way, whether they were men or women, he might bring them bound unto Jerusalem.

3 And as he journeyed, he came near Damascus: and suddenly there shined round about him a light from heaven:

4 And he fell to the earth, and heard a voice saying unto him, Saul, Saul, why persecutest thou me?

5 And he said, Who art thou, Lord? And the Lord said, I am Jesus whom thou persecutest: it is hard for thee to kick against the pricks.

6 And he trembling and astonished said, Lord, what wilt thou have me to do? And the Lord said unto him, Arise, and go into the city, and it shall be told thee what thou must do.

7 And the men which journeyed with him stood speechless, hearing a voice, but seeing no man.

8 And Saul arose from the earth; and when his eyes were opened, he saw no man: but they led him by the hand, and brought him into Damascus.

9 And he was three days without sight, and neither did eat nor drink.

10 And there was a certain disciple at Damascus, named Ananias; and to him said the Lord in a vision, Ananias. And he said, Behold, I am here, Lord.

11 And the Lord said unto him, Arise, and go into the street which is called Straight, and enquire in the house of Judas for one called Saul, of Tarsus: for, behold, he prayeth,

12 And hath seen in a vision a man named Ananias coming in, and putting his hand on him, that he might receive his sight.

13 Then Ananias answered, Lord, I have heard by many of this man, how much evil he hath done to thy saints at Jerusalem:

14 And here he hath authority from the chief priests to bind all that call on thy name.

15 But the Lord said unto him, Go thy way: for he is a chosen vessel unto me, to bear my name before the Gentiles, and kings, and the children of Israel:

16 For I will shew him how great things he must suffer for my name’s sake.

17 And Ananias went his way, and entered into the house; and putting his hands on him said, Brother Saul, the Lord, even Jesus, that appeared unto thee in the way as thou camest, hath sent me, that thou mightest receive thy sight, and be filled with the Holy Ghost.

18 And immediately there fell from his eyes as it had been scales: and he received sight forthwith, and arose, and was baptized.

19 And when he had received meat, he was strengthened. Then was Saul certain days with the disciples which were at Damascus.

20 And straightway he preached Christ in the synagogues, that he is the Son of God.

21 But all that heard him were amazed, and said; Is not this he that destroyed them which called on this name in Jerusalem, and came hither for that intent, that he might bring them bound unto the chief priests?

22 But Saul increased the more in strength, and confounded the Jews which dwelt at Damascus, proving that this is very Christ.

23 And after that many days were fulfilled, the Jews took counsel to kill him:

24 But their laying await was known of Saul. And they watched the gates day and night to kill him.

Later, he wrote because he desired that his fellow Jews be saved, those of the nation of Israel. He wrote from his own life; he wrote his testimony.  Before being born again, he had been one of them in his efforts to establish his righteousness before God.  You might say that he wrote from a “been there, done that” experience.

Romans 10:1 Brethren, my heart’s desire and prayer to God for Israel is, that they might be saved.

2 For I bear them record that they have a zeal of God, but not according to knowledge.

3 For they being ignorant of God’s righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God.

4 For Christ is the end of the law for righteousness to every one that believeth.

5 For Moses describeth the righteousness which is of the law, That the man which doeth those things shall live by them.

6 But the righteousness which is of faith speaketh on this wise, Say not in thine heart, Who shall ascend into heaven? (that is, to bring Christ down from above:)

7 Or, Who shall descend into the deep? (that is, to bring up Christ again from the dead.)

8 But what saith it? The word is nigh thee, even in thy mouth, and in thy heart: that is, the word of faith, which we preach;

9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.

10 For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.

11 For the scripture saith, Whosoever believeth on him shall not be ashamed.

12 For there is no difference between the Jew and the Greek: for the same Lord over all is rich unto all that call upon him.

13 For whosoever shall call upon the name of the Lord shall be saved.