Paul says he was once a blasphemer and injurious, meaning he cause torment to others.
Being as a zealous Jew for the Law and a Pharisee, he likely said and did many bad and terrible things about and towards Christ and the people of God.
Acts 22
“Men, brethren, and fathers, hear ye my defense which I make now unto you.”
2 (And when they heard that he spoke in the Hebrew tongue to them, they were the more silent.) And he said,
3 “I am verily a man who am a Jew, born in Tarsus, a city of Cilicia, yet brought up in this city at the feet of Gamaliel, and taught according to the perfect manner of the law of the fathers, and was zealous toward God, as ye all are this day.
4 And I persecuted this Way unto the death, binding and delivering into prisons both men and women,
5 as also the high priest doth bear me witness and all the council of the elders. From them I also received letters unto the brethren, and I went to Damascus to bring those who were there bound unto Jerusalem to be punished.
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He was however God's chosen vessel to carry the word of the Lord to many peoples, he just didn't yet know it or God.
Paul puts up his own life story as an encouragement to all who would come after him and believe. Paul would have persecuted you all as believers unto the death, binding, delivering into prisons both men and women. He was a dangerous man to be around.
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12 And I thank Christ Jesus our Lord, who hath enabled me, that He counted me faithful, putting me into the ministry—
13
I, who was once a blasphemer and a persecutor and injurious. But I obtained mercy, because I did it ignorantly in unbelief;
14 and the grace of our Lord was exceedingly abundant with faith and love, which are in Christ Jesus.
15 This is a faithful saying and worthy of all acceptance: that Christ Jesus came into the world to save sinners, of whom I am chief.
16
Nonetheless, for this cause I obtained mercy, that in me first Jesus Christ might show forth all longsuffering as a pattern for those who should hereafter believe in Him to life everlasting.
17 Now unto the King eternal, immortal, invisible, the only wise God, be honor and glory for ever and ever. Amen.
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Paul, an apostle of Jesus Christ by the commandment of God our Savior, and Lord Jesus Christ, who is our hope, Unto Timothy, my own son in the faith: Grace, mercy, and peace from God our Father and Jesus Christ our Lord. As I besought thee when I went into Macedonia to abide still at Ephesus...
classic.biblegateway.com
Paul, an apostle of Jesus Christ by the commandment of God our Savior, and Lord Jesus Christ, who is our hope,
2 Unto Timothy, my own son in the faith: Grace, mercy, and peace from God our Father and Jesus Christ our Lord.
3 As I besought thee when I went into Macedonia to abide still at Ephesus, that thou mightest charge some that they teach no other doctrine,
4 neither give heed to fables and endless genealogies, which promote questions rather than godly edifying in the faith, so do!
5 Now the aim of the commandment is charity out of a pure heart, with a good conscience and with faith unfeigned,
6 from which some have swerved and have turned aside unto vain jangling,
7 desiring to be teachers of the law, understanding neither what they say nor whereof they affirm.
8 But we know that the law is good if a man use it lawfully,
9 knowing this: that the law is not made for a righteous man, but for the lawless and disobedient, for the ungodly and for sinners, for the unholy and profane, for murderers of fathers and murderers of mothers, for manslayers,
10 for whoremongers, for those who defile themselves with mankind, for menstealers, for liars, for perjurers, and for whatever else is contrary to sound doctrine
11 according to the glorious Gospel of the blessed God, which was committed to my trust.
12 And I thank Christ Jesus our Lord, who hath enabled me, that He counted me faithful, putting me into the ministry—
13
I, who was once a blasphemer and a persecutor and injurious. But I obtained mercy, because I did it ignorantly in unbelief;
14 and the grace of our Lord was exceedingly abundant with faith and love, which are in Christ Jesus.
15 This is a faithful saying and worthy of all acceptance: that Christ Jesus came into the world to save sinners, of whom I am chief.
16 Nonetheless, for this cause I obtained mercy, that in me first Jesus Christ might show forth all longsuffering as a pattern for those who should hereafter believe in Him to life everlasting.
17 Now unto the King eternal, immortal, invisible, the only wise God, be honor and glory for ever and ever. Amen.
18 This charge I commit unto thee, son Timothy, according to the prophecies which went before concerning thee, that thou by them mightest wage a good warfare,
19 holding faith and a good conscience which some, having put aside, have suffered shipwreck concerning faith.
20 Among them are Hymenaeus and Alexander, whom I have delivered unto Satan, that they may learn not to blaspheme.
