1:1 Paul, called by the will of God to be an apostle of Christ Jesus, and our brother Sosthenes,
2 To the church of God that is in Corinth, to those sanctified in Christ Jesus, called to be saints together with all those who in every place call upon the name of our Lord Jesus Christ, both their Lord and ours:
3 Grace to you and peace from God our Father and the Lord Jesus Christ.
1 Corinthians 1:1-3 (ESV)
Paul packs so much into an introduction. It’s kind of amazing. He talks about his call and apostleship. This is why we find his letter authoritative for the church, because he is an apostle. An apostle wasn’t someone who just decided to go speak about God. Apostle had a different meaning in antiquity than the narrow meaning we give it today. Anyone could hire an apostle. It was somewhat like hiring an attorney and giving him the authority to speak on your behalf. But that is what an apostle was, someone commissioned to speak for you on your behalf and with your authority. We would say power of attorney. What Paul is saying is that he has the authority to speak, not about God, but for Christ. That Paul has this authority is backed up on numerous occasions when the other apostles recognize his apostleship even so much as to acquiesce to his judgment and accept his rebuke. So as an apostle he writes to the church of God in Corinth.
This is a complicated bit of phraseology and it still causes some head scratching. When Paul talks about the church in Corinth he does so with deliberate ambiguity. He uses a term that can mean congregation as we think of a congregation and can mean equally the church of God that is found everywhere. For Paul there was no division between an individual congregation and the rest of the body of Christ known as the church, but the church manifested itself in each congregation. What went for one congregation went for the whole church of God and vice versa. For this reason what Paul writes here applies to us today as much as it did then, though applying the principle in our day.
The Church is one because all the sanctified have one Lord Jesus Christ. The letter of First Corinthians is known to be one of the sharpest letters Paul ever wrote. He chastises the Corinthians hard for all sorts of sinful behavior. Today I read a lot of different Christian authors and they tend to be rather quick to say so and so isn’t Christian if they do this, if they believe that. And it is certainly true that much of what Christians do and believe is not in line with the Christian faith. Even as much of what the Corinthians had been led to believe by itinerant preachers wasn’t in line with the Christian faith. Paul has to chastise them concerning the resurrection itself! When a person reads chapter 6 it is worthwhile to keep in mind that Paul wouldn’t be writing that if he didn’t think members hearing it on Sunday morning weren’t actually guilty of doing those things as recently as Saturday night. Yet he still calls them sanctified, still addresses them as Christians because they are baptized. I think there is much to be learned from that in modern discourse.