1 Cor 1:17-18, 23-24, 2 Tim 3:15, Acts 17:2-3, Acts 19:8-10
What Paul taught in each church was that Jesus had accomplished the work of Christ on the cross. (1 Cor 1:17-18,23-24)
1 Cor 1:17 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. 18 For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God. 23 But we preach Christ crucified, unto the Jews a stumblingblock, and unto the Greeks foolishness; 24 But unto them which are called, both Jews and Greeks, Christ the power of God, and the wisdom of God.
Paul explained to Timothy that the Old Testament describes salvation through Christ and that Christ is Jesus. (2 Tim 3:15)
2 Tim 3:15 And that from a child thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus.
So Paul taught in each church that Jesus was the Christ prophesied in the Old Testament. (1 Cor 4:17, Acts 17:2-3, Acts 19:8-10)
1 Cor 4:17 For this cause have I sent unto you Timotheus, who is my beloved son, and faithful in the Lord, who shall bring you into remembrance of my ways which be in Christ, as I teach every where in every church.
Acts 17:2 And Paul, as his manner was, went in unto them, and three sabbath days reasoned with them out of the scriptures, 3 Opening and alleging, that Christ must needs have suffered, and risen again from the dead; and that this Jesus, whom I preach unto you, is Christ.
Acts 19:8 And he went into the synagogue, and spake boldly for the space of three months, disputing and persuading the things concerning the kingdom of God. 9 But when divers were hardened, and believed not, but spake evil of that way before the multitude, he departed from them, and separated the disciples, disputing daily in the school of one Tyrannus. 10 And this continued by the space of two years; so that all they which dwelt in Asia heard the word of the Lord Jesus, both Jews and Greeks.