Paul names himself at the start
The opening sentence gives the clearest answer. Paul identifies himself as an apostle of Jesus Christ, called by God, and addresses the church at Corinth (1 Corinthians 1:1-2). This was not an anonymous document later attached to Paul’s name. From its first line, the letter presents itself as a message from him.
Paul had a close history with this city. Acts describes him staying in Corinth for a considerable time, teaching God’s word and helping establish the church there (Acts 18:9-11). So when serious questions and painful divisions arose among the believers, Paul wrote as their founding teacher and spiritual father in the faith.
Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother,
What about Sosthenes?
Sosthenes appears beside Paul in the greeting as “our brother” (1 Corinthians 1:1). That likely means he was with Paul and joined in sending the letter. Yet Paul is the person who consistently speaks, gives instruction, recalls his own ministry, and exercises apostolic responsibility throughout 1 Corinthians.
Acts also mentions a man called Sosthenes in Corinth during a conflict before the Roman deputy Gallio (Acts 18:17). He may be the same person named in the letter, though Scripture does not explicitly say so. It is wise not to claim more than the text tells us.
A real letter to a real church
Paul wrote because the Corinthian believers faced very practical problems. People in the church were forming rival groups around different leaders, and Paul appealed to them to be united in Christ rather than divided by personality or status (1 Corinthians 1:10-13). He also answered questions they had sent him about marriage, worship, spiritual gifts, generosity, and the resurrection.
The letter carries urgency, but also deep care. Paul did not give up on this messy congregation. He kept pointing them back to Christ crucified, the source of their hope and wisdom (1 Corinthians 1:23-24). That pastoral purpose helps explain why 1 Corinthians feels so direct, personal, and relevant.
Now I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing, and that there be no divisions among you; but that ye be perfectly joined together in the same mind and in the same judgment.
Paul personally confirms the closing
Near the end, Paul notes that he plans to remain at Ephesus for a time, which places him away from Corinth while he writes (1 Corinthians 16:8). Then he adds a greeting written with his own hand (1 Corinthians 16:21). In the ancient world, a writer could use someone else to help physically write a letter, but this personal sign confirmed Paul’s connection with and approval of it.
So the simple answer is Paul. Read 1 Corinthians as a letter from an apostle who knew this church, loved them enough to correct them, and trusted that God could shape them into a faithful community.
The salutation of me Paul with mine own hand.