Both expressions refer to the Lord Jesus. As “the last Adam”, although head of a race, Christ stands alone; as “the second man” He is the pattern for others. Let me seek to explain this.
In 1 Cor. 15: 45 the comparison and contrast is between Adam and Christ as the heads of two different races of men, for the apostle had already said in v22 “For as in the Adam all die, thus also in the Christ all shall be made alive”. However, the terms used in v45 must be carefully noted: “Thus also it is written, The first man Adam became a living soul; the last Adam a quickening spirit”. Paul does not speak of the ‘first man’, a choice of words which would have been sufficient to identify Adam as a person, but he uses the term ‘the first man Adam’. Now Adam is a Hebrew word meaning man or mankind (rather than man as an individual––for which a separate word is employed). “The first man Adam” is Adam as the head of an order of men. When the apostle goes on to speak of Christ, he describes Him, not as the ‘last man’ (for others have been born into the world since) but as “the last Adam”––again, the head of an order of men. Thus the terms “first man Adam” and “last Adam” while referring to Adam and Christ personally, do so as the respective heads of two races of men.
Now of Adam it is said he “became a living soul”, but Christ is referred to simply as “a quickening spirit”. You will notice that the word “became” is omitted in regard to Christ. This is because while the Lord became the last Adam, He always could give life (quicken) and was thus always a quickening spirit. As such He stands alone. The first Adam became a living soul, for God “breathed into his nostrils the breath of life” (Gen. 2: 7)—he was created. By contrast, the last Adam, although Man, is also the Creator, which He must be if He is a quickening spirit.
Now Adam, while marked out head of his race before the fall (as being the first man) was really only head after he had sinned and had subsequently produced offspring. Similarly, Christ, though ever a quickening spirit, only became head of His race in resurrection, for it was as raised from the dead that He breathed into His disciples and said to them “Receive [the] Holy Spirit” (John 20: 22). As thus quickened they became part of the new race. Prior to this the Lord had abode alone: “Except the grain of wheat falling into the ground die, it abides alone; but if it die, it bears much fruit” (John 12: 24). Now the crucifixion and death of Christ terminated the first order of man in God’s eyes—Adam’s race, the natural man. God has begun anew in the risen Man. Hence Christ is the last Adam. God has reached finality in Christ. There is no other order of man to come, the last Adam tells me it is final and conclusive.
Up to now, the apostle has contrasted the vast difference between the two heads, describing one as a “living soul” and the other as a “quickening spirit” (1 Cor. 15: 45). In verse 47, however, Paul changes his terminology and speaks of the source of each of the heads as characterising not only the heads but also the members of the respective races under each headship: “the first man out of [the] earth, made of dust; the second man, out of heaven. Such as he made of dust, such also those made of dust; and such as the heavenly [one], such also the heavenly [ones]” (vs 47, 48). This is why the word “Adam” is dropped and we just read of the “first man” and the “second man”. They are now viewed as distinct pattern men. The terms “first man” and “second man” are used because although countless men have lived and died, characteristically there only ever were just two men: Adam and Christ (Cain was not the second man––he was only a reproduction of the first). The first man was characterised as being “out of [the] earth”, the second Man as “out of heaven”. Again, note the accuracy of the language. While Adam is said also to be “made of dust”, no such corresponding phrase with the words “made of” could be applied to Christ. The source characterises each man: Adam earthly, Christ heavenly. What characterises the heads also characterises the members. As belonging to the first man, we inherited his condition (v48); as belonging now to the second Man “we shall be like him” (1 John 3: 2) or as Paul says here “we shall bear also the image of the heavenly [one]” (1 Cor. 15: 49).
In summary, as the last Adam Christ is viewed as Head of a race but personally unique with none to succeed Him; as the second Man He is viewed as the Head whose characteristics others will bear.
