Barth ( CD 1.2) gives an powerful exposition of the crucial importance of anhypostasis and enhypostasis as Christological concepts. The first refers to the “impersonal” character of the human nature, and the second to the notion that the human nature exists only as the human nature of the Son.

Barth says this:

“Anhypostasis asserts the negative. Since in virtue of the egeneto , i.e., in virtue of the assumptio , Christ’s human nature has its existence - the ancients said, its subsistence - in the existence of God, meaning in the mode of being . . . of the Word, it does not possess it in and for itself, in abstracto . Apart from the divine mode of being whose existence it acquires it has none of its own;l i.e., apart from its concrete existence in God in the event of the unio , it has no existence of its own, it is anhypostatos . Enhypostasis asserts the positive. In virtue of the egeneto , i.e., in virtue of the assumptio , the human nature acquires existence (subsistence) in the existence of God, meaning in the mode of being . . . of the Word. This divine mode of being gives it existence in the event of the unio , and in this way it has a concrete existence of its own, it is enhypostatos .”

Without these two claims, the incarnation is lost. You lose either the divinity, or the humanity, or the union. You can lose the divinity because you lose the divine initiative and Lordship; a separately subsistent human nature is not the human nature specifically of the Word. You lose the humanity for the same reason, because the humanity is in danger of becoming a second principle cooperating with the divine nature, and thus is not truly human, i.e., creaturely and receptive, but in a mutual relation with the Word. You lose the union most obviously because without the en and the an, you have “a double existence of Christ as Logos and as Man.”

Barth also addresses the “primitive” modern objection that an impersonal humanity is not fully human: “This argument is primitive because it rests simply upon a misunderstanding of the Latin impersonalitas used occasionally for anhypostasis . But what Christ’s human nature lacks according to the early doctrine is not what we call personality. This the early writers called individualitas , and they never taught that Christ’s human nature lacked this, but rather that this qualification actually belonged to true human being. Personalitas was their name for what we call existence or being. Their negative position asserted that Christ’s flesh in itself has no existence, and this was asserted in the interests of their positive position that Christ’s flesh has its existence through the Word and in the Word, who is God Himself acting as Revealer and Reconciler.”