Jamieson, Fausset & Brown

seeing it is a righteous thing--This justifies the assertion above of there being a "righteous judgment" (Th2 1:5), namely, "seeing that it is (literally, 'if at least,' 'if at all events it is') a righteous thing with (that is, in the estimation of) God" (which, as we all feel, it certainly is). Our own innate feeling of what is just, in this confirms what is revealed. recompense--requite in kind, namely, tribulation to them that trouble you (affliction to those that afflict you); and to you who are troubled, rest from trouble.