[1 Cor. 12:13] says, “We were all baptized into one body.” The “we all” transcends the local church at Corinth for a couple of reasons. (1) The addressees of the epistle imply or designate a wider audience than the local church at Corinth (1 Cor 1:2). The latter part of the clause, “with all who in every place call upon the name of our Lord Jesus Christ,” seems to enlarge the address of the letter, as does a similar clause in 2 Corinthians 1:1 (“To the church of God which is at Corinth with all the saints who are throughout Achaia”). (2) Paul was undoubtedly not a member of the church at Corinth at the time of writing, yet he and those to whom he was writing were members of the same body (see similar instances of “we” in Rom 12:5 and 1 Thess 4:17, where Paul was almost certainly not a member of the churches to which he was writing).
Rolland McCune, A Systematic Theology of Biblical Christianity, Vol. 3, p. 208.