From Anstey’s Doctrinal Definitions:
This term denotes what is natural and earthy in the human race (1 Cor. 15:47). In Scripture, the “first man” is never said to be corrupt or sinful, because what is inherently natural and soulish in man is not evil; it is what God created. For this reason, the “first man” is not said to be “crucified” with Christ, as is the case with the “old man” (Rom. 6:6).
The creation of man after the first order has many characteristics of God Himself, for he was created in His “image” and after His “likeness” (Gen. 1:26). For instance, man has a definite personality with likes and dislikes. He also has feelings and reasoning powers, etc. These natural qualities are not evil, but are part of the makeup of a human being. God has not brought this into judgment, for it came forth from His own hand in creation. It is the same with our bodies; they are never said to be evil. (The KJV translates Philippians 3:21, “Our vile bodies,” but this is not the best translation, for what God has created is not vile—in the modern sense of the word. If our bodies were vile in that sense, we would never be asked to present them to God as a living sacrifice, as stated in Romans 12:1. “The body of sin,” mentioned in Romans 6:6, is not speaking of our physical bodies, but of the totality of sin as a system. We use the word “body” similarly in other topics. For instance, we will say, “the body of a river,” or “the body of medical knowledge,” etc.)
While what is natural in the “first man” is not said to come under the judgment of God, that whole order of manhood has been superseded by another order of manhood under Christ that is superior. This is the force of the word “afterward” in 1 Corinthians 15:46. Hence, the first man has been set aside and replaced by the new order of manhood under Christ. Christians are part of that new creation race now (2 Cor. 5:17; Eph. 2:10) and are waiting to bear the image of “the second man” physically, when they are glorified (1 Cor. 15:49).
The term “first man” is often used inter-changeably with the term “old man,” but as we have noted, these terms are not synonymous. The Apostle Paul uses these terms to define two different aspects of the human race under Adam. The “old man” denotes the corrupt state of the fallen race, whereas the “first man” denotes what is natural and earthy in the race. The first man has been superseded, but the old man has been judged. (See Old Man.)