1 Now I, Paul, myself implore you by the meekness and gentleness of Christ—I who in presence am base among you, but being absent am bold toward you. 2 But I implore you that when I am present I may not be bold with the confidence with which I intend to be bold against some, who think of us as if we walked according to the flesh. 3 For though we walk in the flesh, we do not war according to the flesh. 4 For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strongholds, 5 Casting down imaginations and every high thing that exalts itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ, 6 And having in a readiness to avenge all disobedience, when your obedience is fulfilled.
7 Do you look at things after the outward appearance? If any man trusts to himself that he is Christ's, let him of himself think this again, that as he is Christ's, even so are we Christ's. 8 For though I should boast somewhat more of our authority, which the Lord has given us for edification and not for your destruction, I should not be ashamed, 9 So that I may not seem as if I would terrify you by letters. 10 "For his letters," they say, "are weighty and powerful, but his bodily presence is weak and his speech contemptible." 11 Let such a one think this: that what we are in word by letters when we are absent, such we will also be in deed when we are present.
12 For we dare not make ourselves of the number, or compare ourselves with some who commend themselves. But they, measuring themselves by themselves and comparing themselves among themselves, are not wise. 13 But we will not boast of things without our measure, but according to the measure of the rule which God has distributed to us, a measure to reach even to you. 14 For we do not stretch ourselves beyond our measure, as though we did not reach to you. For we have come as far as to you also in preaching the gospel of Christ, 15 Not boasting of things without our measure, that is, of other men's labors, but having hope, when your faith is increased, that we shall be enlarged by you, according to our rule abundantly, 16 To preach the gospel in the regions beyond you, and not to boast in another man's line of things already prepared. 17 But "he who glories, let him glory in the Lord." 18 For he who commends himself is not approved, but whom the Lord commends.
[v.3-6] - Reference, Ephesians 6:12.
[v.17] - Quoting Jeremiah 9:24 (paraphrased). See also the comment for 1st Corinthians 1:31.