Jamieson, Fausset and Brown on Romans 12:3

3. For I say--authoritatively

through the grace given unto me--as an apostle of Jesus Christ; thus exemplifying his own precept by modestly falling back on that office which both warranted and required such plainness towards all classes.

to every man that is among you, not to think, &c.--It is impossible to convey in good English the emphatic play, so to speak, which each word here has upon another: "not to be high-minded above what he ought to be minded, but so to be minded as to be sober-minded" [Calvin, Alford]. This is merely a strong way of characterizing all undue self-elevation.

according as God hath dealt to every man the measure of faith--Faith is here viewed as the inlet to all the other graces, and so, as the receptive faculty of the renewed soul--that is, "as God hath given to each his particular capacity to take in the gifts and graces which He designs for the general good."