1:1-3 Paul begins his letter to the church in Corinth by affirming his calling as an apostle of Christ Jesus. And that appointment came by the will of God (1:1-2), when the resurrected Lord appeared to Paul and made him his chosen instrument to proclaim his name (see Acts 9:1-30).
1:4-8 In spite of the church’s many problems, Paul actually gave thanks for the believers because he was confident they had experienced God’s grace, the basis of salvation (1:4). Because they had been saved, they had been enriched by God in every way, particularly in the spiritual [gifts] they had received, which gave testimony about the validity of Christ’s work in their midst (1:5-7). So even though they had misused their spiritual gifts (something that would be addressed later; see 12:1–14:40), Paul was confident that they would stand blameless on judgment day because of the finished work of Jesus Christ for them (1:8).
1:9 God is faithful to keep his promises to save us through our faith in Christ. As a result of our salvation, we have access to fellowship with Christ. But we are called to live out our fellowship with our Savior through fellowship with his saints. Sadly, the prevalent sins in the church were hindering such fellowship. And this is why Paul needed to write to them.