A.T. Robertson Commentary 2 Corinthians 13

The third time I am coming (τριτον ερχομα). Either the third that he had planned to come or that he had been twice. The warning is made by quoting De 19:15.

As when I was present the second time (ως παρων το δευτερον). This translation assumes the second visit as already made. It is a natural way to take the Greek ως παρων. But ως with παρων can also mean "as if present" the second time (Authorized Version). Probably "as when" is the more natural rendering, but the other cannot be ruled entirely out in view of 1:15-23.

If I come again (εαν ελθω εις το παλιν). Condition of third class. The use of παλιν of itself suits the idea that Paul had not yet made the second visit as it means simply "again" or "back," but in Mt 26:44 we find παλιν εκ τριτου (again a third time) and so it is not decisive.

A proof of Christ (δοκιμην του Χριστου). He will give it to them. "I will not spare." He will show that Christ speaks "in me" (εν εμο).

But we shall live with him through the power of God (αλλα ζησομεν συν αυτω εκ δυναμεως θεου). So real is Paul's sense of his union with Christ.

Unless indeed ye be reprobate (ε μητ αδοκιμο εστε). Paul challenged his opposers in Corinth to try (πειραζετε) themselves, to test (δοκιμαζετε) themselves, whether they were "in the faith" (εν τη πιστε), a much more vital matter for them than trying to prove Paul a heretic. Such tests can be made, unless, alas, they are "reprobate" (αδοκιμο, the very adjective that Paul held up before himself as a dreadful outcome to be avoided, 1 Corinthians 9:27).