For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.
Note 27 at Ro 8:13: Paul was speaking of death in a figurative sense rather than a literal sense. He was addressing believers who had already received eternal life through the new birth (see note 2 at Joh 3:3), and he was not saying that they would lose their salvation if they walked "after" the flesh (see note 10 at Ro 8:4).
As explained in note 3 at Ro 6:23, "death" is a term that not only refers to physical death but also can denote all the effects of sin in our lives. Therefore, Paul was speaking about experiencing defeat as we Christians walk after the flesh compared to experiencing victory when we walk after the Spirit.
Note 28 at Ro 8:13: The word "mortify" was translated from the Greek word "THANATOO," and it means "to kill" (Strong's Concordance). The American Heritage Dictionary defines "mortify" as "to discipline (one's body and appetites) by self-denial." If we deaden ourselves to the flesh by self-denial and follow the leadership of the Holy Spirit, we will live.