A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Corinthians 13:1
co2 13:1HE THREATENS A SEVERE PROOF OF HIS APOSTOLIC AUTHORITY, BUT PREFERS THEY WOULD SPARE HIM THE NECESSITY FOR IT. (Co2 13:1-14)
This is the third time I am coming to you--not merely preparing to come to you. This proves an intermediate visit between the two recorded in Act 18:1; Act 20:2.
In the mouth of two or three witnesses shall every word be established--Quoted from Deu 19:15, Septuagint. "I will judge not without examination, nor will I abstain from punishing upon due evidence" [CONYBEARE and HOWSON]. I will no longer be among you "in all patience" towards offenders (Co2 12:12). The apostle in this case, where ordinary testimony was to be had, does not look for an immediate revelation, nor does he order the culprits to be cast out of the church before his arrival. Others understand the "two or three witnesses" to mean his two or three visits as establishing either (1) the truth of the facts alleged against the offenders, or (2) the reality of his threats. I prefer the first explanation to either of the two latter.
2 Corinthians 13:2
co2 13:2Rather, "I have already said (at my second visit), and tell you (now) beforehand, AS (I did) WHEN I WAS PRESENT THE SECOND TIME, SO also NOW in my absence (the oldest manuscripts omit the 'I write,' which here wrongly follows in English Version Greek text) to them which heretofore have sinned (namely, before my second visit, Co2 12:21), and to all others (who have sinned since my second visit, or are in danger of sinning)." The English Version, "as if I were present the second time," namely, this next time, is quite inconsistent with Co2 13:1, "this is the third time I am coming to you," as Paul could not have called the same journey at once "the second" and "the third time" of his coming. The antithesis between "the second time" and "now" is palpable.
if I come again, &c.--that is, whensoever I come again (Act 20:2). These were probably the very words of his former threat which he now repeats again.
2 Corinthians 13:3
co2 13:3Since--The reason why he will not spare: Since ye challenge me to give a "proof" that Christ speaks in me. It would be better if ye would "prove your own selves" (Co2 13:5). This disproves the assertion of some that Scripture nowhere asserts the infallibility of its writers when writing it.
is not weak--in relation to you, by me and in this very Epistle, in exercising upon you strong discipline.
mighty in you--has given many proofs of His power in miracles, and even in punishing offenders (Co2 5:11, Co2 5:20-21). Ye have no need to put me to the proof in this, as long ago Christ has exhibited great proofs of His power by me among you (Co2 12:12) [GROTIUS]. It is therefore not me, but Christ, whom ye wrong: it is His patience that ye try in despising my admonitions, and derogating from my authority [CALVIN].
2 Corinthians 13:4
co2 13:4though--omitted in some of the oldest manuscripts; then translate, "For He was even crucified," &c.
through weakness--Greek, "from weakness"; that is, His assumption of our weakness was the source, or necessary condition, from which the possibility of His crucifixion flowed (Heb 2:14; Phi 2:7-8).
by--Greek, "from"; "owing to."
the power of God--the Father (Rom 1:4; Rom 6:4; Eph 1:20).
weak in him--that is, in virtue of our union with Him, and after His pattern, weakness predominates in us for a time (exhibited in our "infirmities" and weak "bodily presence," Co2 10:10; Co2 12:5, Co2 12:9-10; and also in our not putting into immediate exercise our power of punishing offenders, just as Christ for a time kept in abeyance His power).
we shall live with him--not only hereafter with Him, free from our present infirmities, in the resurrection life (Phi 3:21), but presently in the exercise of our apostolic authority against offenders, which flows to us in respect to you from the power of God, however "weak" we now seem to you. "With Him," that is, even as He now exercises His power in His glorified resurrection life, after His weakness for a time.
2 Corinthians 13:5
co2 13:5Examine--Greek, "Try (make trial of) yourselves."
prove your own selves--This should be your first aim, rather than "seeking a proof of Christ speaking in me" (Co2 13:3).
your own selves--I need not speak much in proof of Christ being in me, your minister (Co2 13:3), for if ye try your own selves ye will see that Christ is also in you [CHRYSOSTOM], (Rom 8:10). Finding Christ dwelling in yourselves by faith, ye may well believe that He speaks in me, by whose ministry ye have received this faith [ESTIUS]. To doubt it would be the sin of Israel, who, after so many miracles and experimental proofs of God's presence, still cried (Exo 17:7), "Is the Lord among us or not?" (Compare Mar 8:11).
except ye be reprobates--The Greek softens the expression, "somewhat reprobates," that is, not abiding the "proof" (alluding to the same word in the context); failing when tested. Image from metals (Jer 6:30; Dan 5:27; Rom 1:28).
2 Corinthians 13:6
co2 13:6we . . . not reprobates--not unable to abide the proof to which ye put us (Co2 13:6). "I trust that" your own Christianity will be recognized by you (observe, "ye shall know," answers to "know your own selves," Co2 13:5) as sufficient "proof" that ye are not reprobates, but that "Christ speaks in me," without needing a proof from me more trying to yourselves. If ye doubt my apostleship, ye must doubt your own Christianity, for ye are the fruits of my apostleship.
2 Corinthians 13:7
co2 13:7I pray--The oldest manuscripts read, "we pray."
not that we should appear approved--not to gain credit for ourselves, your ministers, by your Christian conduct; but for your good [ALFORD]. The antithesis to "reprobates" leads me to prefer explaining with BENGEL, "We do not pray that we may appear approved," by restraining you when ye do evil; "but that ye should do what is right" (English Version, "honest").
though we be as reprobates--though we be thereby deprived of the occasion for exercising our apostolic power (namely, in punishing), and so may appear "as reprobates" (incapable of affording proof of Christ speaking in us).
2 Corinthians 13:8
co2 13:8Our apostolic power is given us that we may use it not against, but for the furtherance of, the truth. Where you are free from fault, there is no scope for its exercise: and this I desire. Far be it from me to use it against the innocent, merely in order to increase my own power (Co2 13:10).
2 Corinthians 13:9
co2 13:9are glad--Greek, "rejoice."
when we are weak--having no occasion for displaying our power; and so seeming "weak," as being compassed with "infirmities" (Co2 10:10; Co2 11:29-30).
ye . . . strong--"mighty" in faith and the fruits of the Spirit.
and--not in the oldest manuscripts.
we wish--Greek, "pray for."
your perfection--literally, "perfect restoration"; literally, that of a dislocated limb. Compare Co2 13:11, "Be perfect," the same Greek word; also in Co1 1:10, "perfectly joined together"; Eph 4:12, "the perfecting of the saints."
2 Corinthians 13:10
co2 13:10Therefore--because I wish the "sharpness" to be in my letters rather than in deeds [CHRYSOSTOM].
edification . . . not to destruction--for building up . . . not for casting down. To "use sharpness" would seem to be casting down, rather than building up; therefore he prefers not to have to use it.
2 Corinthians 13:11
co2 13:11farewell--meaning in Greek also "rejoice"; thus in bidding farewell he returns to the point with which he set out, "we are helpers of your joy" (Co2 1:24; Phi 4:4).
Be perfect--Become perfect by filling up what is lacking in your Christian character (Eph 4:13).
be of good comfort-- (Co2 1:6; Co2 7:8-13; Th1 4:18).
2 Corinthians 13:14
co2 13:14The benediction which proves the doctrine of the Divine Trinity in unity. "The grace of Christ" comes first, for it is only by it we come to "the love of God" the Father (Joh 14:6). The variety in the order of Persons proves that "in this Trinity none is afore or after other" [Athanasian Creed].
communion--joint fellowship, or participation, in the same Holy Ghost, which joins in one catholic Church, His temple, both Jews and Gentiles. Whoever has "the fellowship of the Holy Ghost," has also "the grace of our Lord Jesus Christ," and "the love of God"; and vice versa. For the three are inseparable, as the three Persons of the Trinity itself [CHRYSOSTOM]. The doctrine of the Trinity was not revealed clearly and fully till Christ came, and the whole scheme of our redemption was manifested in Him, and we know the Holy Three in One more in their relations to us (as set forth summarily in this benediction), than in their mutual relations to one another (Deu 29:29).
Amen--omitted in the oldest manuscripts. Probably added subsequently for the exigencies of public joint worship.