"Adam, who is a type of Him who was to come"
That Adam and Christ are heads and representatives of human beings is a fundamental truth. Foundational to this truth fact is "union." All human beings have a union with Adam and Eve, especially Adam. Likewise all the elect (or all believers, or all saved people) have a union with Christ, "the second Adam" or "last Adam." (See Rom. 5: 14; I Cor. 15: 45) Paul says that Adam is a type of Christ and one of the chief ways both Adam and Christ are alike is in their status as "federal heads" of their respective peoples. Adam is the head and representative of every human being, of everyone who has descended from him. The second Adam is the head and representative of all those from among the human race who are united to him by faith. Physical birth unites humans with Adam. Spiritual birth, the second or new birth, unites humans with the second Adam.
It is a well known fact that "in Christ" or "with Christ" are ways in which union with Christ is described. It is described as a person being "in Christ" as well as Christ being in the believer. Getting into Christ, and his getting into the believer, occurs when a person believes in Christ and asks Christ to come into his life. That is when union begins. This is evident from the words of the apostle Paul when he says - "For as in Adam all die, even so in Christ all shall be made alive." (I Cor. 15: 22 nkjv) "In Adam" and "in Christ" denote union. Water baptism pictures entering into Christ and the Lord's Supper pictures Christ entering into the believer.
There are kinds of union. Theologians speak of several kinds of ways all are in union with Adam. As we will see, there is a sense in which union with Adam begins with physical birth, and another sense in which union occurred at the time Adam was created. In other words, every human being was in some sense "in Adam" when Adam was created. Theologians call this "seminal union." By this they mean that every human comes from Adam's semen (and Eve's egg too of course), or "seed" (the Greek word for seed is sperma, i.e. sperm), which also means that all are but "Adam multiplied." All get their DNA from him and Eve. A text that speaks of seminal union is where Paul says that Levi, before he ever existed, was "in the loins of Abraham." (Heb. 7: 10) Not only that, but Paul says that Levi "paid tithes in Abraham." (vs. 6) That act of paying tithes by Abraham is said to have also been the act of Levi when he only had a seminal existence in Abraham. In like manner we say that every human being was "in Adam" seminally so that when Adam acted in eating of the forbidden tree and sinned, so too did every human being eat that tree and sinned. Let the Pelagians disprove that Levi acted in Abraham or that every man acted in Adam.
There is also what is called "representative union" and a union that involves headship. The headship of Adam and his being constituted by God as the legal head of the human race is implied in the fact that Adam and Christ are alike, and surely this likeness cannot exclude the idea of being the head and representative of groups of people. We read in the scriptures where it is stated by Paul that "the head of every man is Christ, the head of woman is man, and the head of Christ is God." (I Cor. 11: 3 nkjv) The head represents the entity of the body of people to which it is connected. A man is the head of the woman and the family. He represents the family. Christ is "the head of every man." But so too is Adam. They both represent the human race. Christ represents the human race but especially those of the race who are elect, who are believers.
The Case of the Angels
The Lord could have made the human race after the example of the angels. Angels, as all acknowledge, were all created individually. No angel was born of other angels. The angels of heaven do not marry nor have sex, nor have children. (See Matt. 22: 30) Each angel therefore could not be held responsible for what another angel did. We also read of where some angels sinned and were, like the first fallen angel Lucifer (who became the serpent, Satan, or the Devil), cast out of heaven. (See Luke 10: 18; II Peter 2: 4; Jude 1: 6; Rev. 12: 3-4, 7-9) There was no original sin among the angels. God did not impute the sin of Lucifer to the other angels. They were not constituted by God as a self propagating race. Not all the angels sinned. Those who did not sin are the "holy angels" who Paul calls "the elect angels." (I Tim. 5: 21) This of course implies that there are non elect angels; These would be the angels that sinned. So, was there no salvation provided for the angels that sinned? And, if there was, what would that entail? How would the sin of angels be atoned for or propitiated? Notice these words of the apostle: "For verily he took not on him the nature of angels; but he took on him the seed of Abraham." (Heb. 2: 16 kjv)
Barnes in his commentary wrote:
"Margin, "He taketh not hold of angels, but of the seed of Abraham he taketh hold...The general idea is that of seizing upon, or laying hold of anyone - no matter what the object is - whether to aid, or to drag to punishment, or simply to conduct. Here it means to lay hold with reference to "aid," or "help;" and the meaning is, that he did not seize the nature of angels, or take it to himself with reference to rendering "them" aid, but he assumed the nature of man - in order to aid "him." He undertook the work of human redemption, and consequently it was necessary for him to be man."
Most bible teachers agree that the election of some angels was for the purpose of keeping them from sinning and from falling. This being so, the angels not chosen were not kept from apostasy. With man, however, God chose to let him fall into sin and then to redeem those who he likewise chose to salvation. The case of the election of angels was to keep them secure, or immutable, in their holiness and fealty to God. The case of the election of men, however, was to allow them to sin and then to provide forgiveness through Christ and by his becoming a sacrifice and substitute and bearing the penalty for those condemned.
God could have made every man personally and individually as he did the angels and so there would have been no original sin or imputation of one man's sin to another man. But, he did not.
Under "Adam and Federal Headship," A. W. PINK wrote the following things which I fully endorse (See here - emphasis mine):
“It is of vital importance for a right understanding of much in God’s Word to observe the relation which Adam sustained to his posterity. Adam was not only the common parent of mankind, but he was also their federal head and representative. The whole human race was placed on probation or trial in Eden. Adam acted not for himself alone, but he transacted for all who were to spring from him. Unless this basic fact be definitely apprehended, much that ought to be relatively clear to us will be shrouded in impenetrable mystery. Yea, we go further, and affirm that, until the federal headship of Adam and God’s covenant with him in that office be actually perceived, we are without the key to God’s dealings with the human race, we are unable to discern man’s relation to the divine law, and we appreciate not the fundamental principles upon which the atonement of Christ proceeded."
Since Adam is a type of Christ, understanding how his original sin affected the human race is most important. It helps us to understand the person of Christ and how our sins were imputed to him, how he paid the penalty for our sins by being the sinner's substitute.
Pink says further:
“Federal headship” is a term which has almost entirely disappeared from current religious literature—so much the worse for our moderns. It is true that the expression itself does not verbally occur in Scripture; yet like the words Trinity and the divine incarnation, it is a necessity in theological parlance and doctrinal exposition. The principle or fact which is embodied in the term “federal headship” is that of representation. There been but two federal heads: Adam and Christ, with each of whom God entered into a covenant. Each of them acted on behalf of others, each legally represented as definite people, so much so that all whom they represented were regarded by God as being in them. Adam represented the whole human race; Christ represented all those whom the Father had, in His eternal counsels, given to Him."
This seems clear to me to be the teaching of Romans chapter five and First Corinthians chapter fifteen. It is because Adam is a figure of Christ and because Christ is called the second or "last Adam," that they are alike in regard to imputation. They are alike and yet different. Their likeness is chiefly because they are both heads and representatives of mankind. Their differences are seen in the fact that Adam disobeyed God but Christ obeyed God (and never disobeyed); And, is seen in the fact that Adam's disobedience made all he represented sinners (or unrighteous) while Christ's obedience made all he represented to be sinless (or righteous); And, Adam's sin brought condemnation and death to all that he represented, but Christ's obedience and holiness brought justification and life to all that he represented.
Pink says further:
“When Adam stood in Eden as a responsible being before God, he stood there as a federal head, as the legal representative of all his posterity. Hence, when Adam sinned, all for whom he was standing are accounted as having sinned; when he fell, all whom he represented fell; when he died, they died. So too was it with Christ. When He came to this earth, He, too, stood in a federal relationship to His own people; and when He became obedient unto death, all for whom He was acting were accounted righteous; when He rose again from the dead, all whom He represented rose with Him; when He ascended on high, they were regarded as ascending with Him. “For as in Adam all die, even so in Christ shall all be made alive” (1 Cor. 15:22)."