Paul relates that Christ appeared to Cephas and James and "all the apostles." Paul recognizes James and Cephas as the pillars of the "churches of God in Christ Jesus in Judaea." So the appearances of Jesus the Lord occurred first in the churches of God in Judaea, then later they happened to Paul; in the time in between Paul persecuted this same community, which he calls "the church of God."
The phrase "the church of God" (he ekklesia tou theou), or, "the churches of God," is a distinctive usage in authentic Paul. Apart from a casual and vague and possibly inauthentic line in the appendix to Romans ("all the churches of Christ greet you," 16:16), Paul seems never to speak of the church of Christ, of Jesus Christ, or of Christ Jesus. By contrast, other NT authors use the term "church" quite differently, and only use "church of God" for pseudo-Paul (Acts 20:28, 1 Timothy 3:5), but without Paul's usual connotations.
In this thread I am exploring whether Paul's "the church of God" was something definite and specific, both in his mind and in reality, and what we can know or infer about it.
Paul uses "the church of God" to refer to the communities that he himself founded, especially in Corinth:
To the church of God which is at Corinth, to those sanctified in Christ Jesus, called to be saints together with all those who in every place call on the name of our Lord Jesus Christ, both their Lord and ours:
2 Corinthians 1:1
Paul, an apostle of Christ Jesus by the will of God, and Timothy our brother. To the church of God which is at Corinth...
But he also uses it to refer to gatherings or a community that he persecuted during his "former life in Judaism":
For you have heard of my former life in Judaism, how I persecuted the church of God violently and tried to destroy it; 14 and I advanced in Judaism beyond many of my own age among my people, so extremely zealous was I for the traditions of my fathers.
1 Corinthians 15:9
For I am the least of the apostles, unfit to be called an apostle, because I persecuted the church of God.
Philippians 3:6
...as for zeal, persecuting the church ["of God," in a few manuscripts]
We learn from Galatians that the churches of God he once persecuted were in Judaea, and he particularly describes them as churches that were "in Christ." Here Paul implies that the faith he now proclaims, his gospel, is the same as the faith that defined the churches he once persecuted:
I was still unknown by sight to the churches of Judea which were in Christ; but only, they kept hearing, “he who once persecuted us is now preaching the faith which he once tried to destroy.” And they were glorifying God because of me.
However, Paul doesn't just preach the resurrection of Jesus, but also his death for sin "according to the scriptures." He leaves it unclear and unknown whether the other apostles also preached, or believed, or supposed, that Christ died for sins in accordance with scripture. It is possible that they knew or worshipped a Jesus/Christ who was a resurrected Lord, but not one who had died for sins.
In Paul's earliest letter, he specifically praises his flock for becoming imitators of the Judaean churches:
For you, brethren, became imitators of the churches of God in Christ Jesus which are in Judea;
However, what immediately follows these words is the famous interpolated rant against the Jews for killing Jesus (2:14b-16). Up to this point, it appeared that Paul was meaning to praise the Thessalonians for their faithful acceptance of the word of God. Indeed, no other part of this epistle suggests that the Thessalonians had suffered any persecution whatever. Paul's only concern is for their growth in faith and holy living. This raises the question, in what respect did the Thessalonians becoming "imitators" of the church of God in Judaea give joy to Paul? It seems to have something more to do with their faith, or habit of worship, or perhaps their ability to learn from God how to love one another (4:9), or something otherwise positive. I suspect the interpolator wanted to suppress something positive that Paul was saying about the kinship between the churches in Thessaloniki and those in Judaea, and not simply inserting this implausible rant against Jews/Judaeans, and this mention by Paul of a historical Jesus.
Returning to 1 Corinthians, several references fill out the picture of what Paul meant by "the church of God," and suggest that it had some deeper meaning than simply being the christian community that Paul established in Corinth.
Give no offense to Jews or to Greeks or to the church of God,
1 Corinthians 11:16
If any one is disposed to be contentious, we recognize no other practice, nor do the churches of God.
1 Corinthians 11:22
What! Do you not have houses to eat and drink in? Or do you despise the church of God and humiliate those who have nothing? What shall I say to you? Shall I commend you in this? No, I will not.
These cases are interesting. Why is Paul telling his own "church of God" not to give offense to the church of God? Perhaps there is an implicit distinction between the believers as frail sinners on the one hand, and the Holy Spirit-filled community on the other. When it comes to the discussion of women's hair and head coverings in chapter 11, he appeals to "the churches of God" as an ancillary source of authority in addition to his own apostolic authority. This might be the most concrete case in which Paul is referring to the church of God as something not simply identical to his own movement. Whereas Paul almost always refers to the church of God in Judaea as a foil to his own apostolic authority, here he seems to be recognizing them as something definite, external to his own ministry, and relevant to a debate about religious practice.
To sum up, Paul recognized an already existing worship community that he calls the "church of God." He never calls it the Church of Jesus Christ, although at some point prior to Paul's calling these communities experienced visions of the risen Lord. It is possible that these communities had existed for some time before they experienced the Lord Jesus, which would explain why they are known to Paul under the merely Jewish name "the church of God." Although I can only leave this thought as a parting suggestion, it is possible that James and Cephas and the original Judaean churches of God--about whom we know essentially nothing--believed in a resurrected Christ or Jesus, but not one who had died an expiatory death for sins as in the gospel of Paul.