This question is often asked in order to undermine mystery truths revealed to Paul. Do not let it trouble you. Paul himself explains why.
Baptism is Jewish, and Paul was a Hebrew of the Hebrews (Phil 3:5).
Baptisms were part of the law as a purification for sin, and Paul knew the ‘perfect manner of the law’ (Acts 22:3).
Baptism was part of Jewish conversion practices (even for Gentiles), and Paul was ‘zealous of the traditions of my fathers’ (Gal 1:14).
The Twelve apostles were sent to baptize unto repentance and remission in the name of Jesus Christ, and the Lord sent Paul to preach forgiveness of sins through Jesus Christ (Acts 13:38-39; Acts 26:18).
Paul was sent to minister to Jews also, and when he did he “became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law” (1 Cor 9:20).
Most of the recorded baptisms in Paul’s early ministry were among Jews and proselytes (Acts 16:14-15; Acts 18:7-8).
Paul circumcised Timothy, according to the law, “because of the Jews which were in those quarters” (Acts 16:3). He took a vow in Jerusalem for the same reason (Acts 21:21-24).
Why Paul Stopped Baptizing
There are many reasons why Paul would baptize, but the more important question concerning the place of baptism for today would be “why would Paul stop baptizing?”, or “Why did he not baptize all?”
Paul says, “I thank God I baptized none of you “, save a few, and “For Christ sent me not to baptize, but to preach the gospel” (1 Cor 1:17).
John the Baptist baptized all in his ministry to Israel. The Lord commanded his twelve apostles to teach the law and baptize all nations (Matt 28:19-20). Peter’s message was “repent and be baptized in the name of Jesus Christ”.
When Paul says Christ sent me not to baptize, he is confessing that he was not following Matt 28 or Mark 16 in his ministry, nor was he baptizing under the same command given to Peter.
Paul was sent to dispense the gospel of the grace of God to Jew and Gentile alike (Acts 9:15; 1 Cor 12:13).
Though he baptized some, he did not baptize all, because he was sent to preach the glory of the cross, and the Lord told him baptism would make the the cross of none effect.
“For Christ sent me not to baptize, but to preach the gospel: not with wisdom of words, lest the cross of Christ should be made of none effect.” – 1 Cor 1:17
Paul did not keep the law for justification nor did he baptize to obey the Lord. He used his liberty help minister the gospel of grace to both Jew and Gentile.
Paul was not sent to circumcise, but he circumcised Timothy in order to minister to Jews. He does not circumcise Titus so that he could minister with a Gentile (Gal 2:3). He would later write circumcision avails nothing (Gal 5:2-6).
He goes to Jerusalem to minister to his kinsmen (Rom 15:27). He observes Pentecost to minister to his kinsmen (Acts 20:16). Yet, he teaches that one day is not above another and to boast in Christ not observation of days (Gal 4:9-11; Col 2:16).
He did not eat certain meats to minister to his kinsmen (1 Cor 8:13). Yet, he ate with Gentiles and taught that every creature is good to eat (Gal 2:12; 1 Tim 4:4).
He baptized Crispus (the chief ruler of the synagogue) in the name of the Lord Jesus, but he was glad not to baptize others that the Corinthians would know that the power of God unto salvation is in the gospel of Christ, and not in the baptisms of Peter, Apollos, or Paul. (1 Cor 1:12-14).
He baptized some, and did not baptize others. In every case it was for the sake of the gospel of grace (1 Cor 9:22-23).
What Paul Knew
Paul knew the law, the traditions of Israel, and the ministry of the Twelve when he said that Peter, James, and John “added nothing” to him (Gal 2:6).
But Paul knew something those Jewish Baptists did not.
The Lord revealed to him the mystery of the one baptism that by one Spirit believers are identified as one body of Christ (Eph 4:4-6, 1 Cor 12:13). The Lord himself separated Paul to preach this mystery of Christ.
He was sent to preach the gospel to Jews and Gentiles, and he became all things to all men so as to save some with his dispensation of the gospel (1 Cor 9:17-23)
Paul spends his whole ministry making all men see the fellowship of this mystery, and doesn’t give a single instruction about water baptism. It was not required.
In every epistle he rages against the rudiments of the world, worldly ordinances, weak and beggarly elements, the works of the law, and traditions of men. Paul was not persecuted for water baptizing, he was in prison for what he said that replaced it (Eph 6:19-20).
Why did Paul baptize? He was a Jew ministering to Jews the Lord Jesus Christ. Baptism was a part of Jewish conversion practice. He ministered to Jews who wanted to convert. Why wouldn’t he?
Why did Paul stop baptizing?
Paul was sent to Gentiles with a gospel of grace without works, Paul stopped ministering to Jews, the Lord sent Paul not to baptize, and the preaching of the cross makes water baptism of none effect.