Jamieson, Fausset and Brown on Romans 6:3

3. Know ye not, that so many of us as were baptized into Jesus Christ--compare 1Co 10:2.

were baptized into his death?--sealed with the seal of heaven, and as it were formally entered and articled, to all the benefits and all the obligations of Christian discipleship in general, and of His death in particular. And since He was "made sin" and "a curse for us" (2Co 5:21; Ga 5:13), "bearing our sins in His own body on the tree," and "rising again for our justification" (Ro 4:25; 1Pe 2:24), our whole sinful case and condition, thus taken up into His Person, has been brought to an end in His death. Whoso, then, has been baptized into Christ's death has formally surrendered the whole state and life of sin, as in Christ a dead thing. He has sealed himself to be not only "the righteousness of God in Him," but "a new creature"; and as he cannot be in Christ to the one effect and not to the other, for they are one thing, he has bidden farewell, by baptism into Christ's death, to his entire connection with sin. "How," then, "can he live any longer therein?" The two things are as contradictory in the fact as they are in the terms.