For Augustine totus Christus is an important doctine which unites Christology and Ecclesiology by affirming the real connection of Christ, the head, to the Church, his body. The concept has been summarised as “Christ, as head, is always present and active in his body, the church; the church and Christ form one single person.”
Thus one reads the following passage from Augustine's Homilies on the Gospel of John:
Then let us rejoice and give thanks that we are made not only Christians, but Christ. Do you understand, brothers, and apprehend the grace of God upon us? Marvel, be glad, we are made Christ. For if he is the head, we are the members: the whole man is he and we… The fullness of Christ, then, is head and members. Head and members, what is that? Christ and the Church (In. Io. XXI.8).Concerned that the concept that we have no Christ without the Church takes away from the uniqueness of Christ, making him incomplete, I've looked around to find the concept if not the language of totus Christus being used elsewhere in later Protestant thought.
So here is John Owen from Vol. 5 'The Doctrine of Justification by Faith', section viii 'Imputation of the sins of the church unto Christ'. It's great!
This, then, I say, is the foundation of the imputation of the sins of the church unto Christ,-- namely, that he and it are one person; the grounds whereof we must inquire into.
But hereon sundry discourses do ensue, and various inquiries are made,--What a person is? In what sense, and in how many senses, that word may be used? What is the true notion of it? What is a natural person? What a legal, civil, or political person? In the explication whereof some have fallen mistakes. And if we should enter into this field, we need not fear matter enough of debate and altercation. But I must needs say, that these things belong not unto our present occasion; nor is the union of Christ and the church illustrated, but obscured by them.
For Christ and believers are neither one natural person, nor a legal or political person, nor any such person as the laws, customs, or usages of men do know or allow of. They are one mystical person; whereof although there may be some imperfect resemblances found in natural or political unions, yet the union from whence that denomination is taken between him and us is of that nature, and arises from such reasons and causes, as no personal union among men (or the union of many persons) has any concernment in. And therefore, as to the representation of it unto our weak understandings, unable to comprehend the depth of heavenly mysteries, it is compared unto unions of divers kinds and natures.
So is it represented by that of man and wife; not as unto those mutual affections which give them only a moral union, but from the extraction of the first woman from the flesh and bone of the first man, and the institution of God for the individual society of life thereon. This the apostle at large declares, Eph.5:25-32: whence he concludes, that from the union thus represented, "We are members of his body, of his flesh, and of his bones," verse 30; or have such a relation unto him as Eve had to Adam, when she was made of his flesh and bone, and so was one flesh with him. So, also, it is compared unto the union of the head and members of the same natural body, 1 Cor.12:12; and unto a political union also, between a ruling or political head and its political members; but never exclusively unto the union of a natural head and its members comprised in the same expression, Eph.4:15; Col.2:19. And so also unto sundry things in nature, as a vine and its branches, John 15:1,2. And it is declared by the relation that was between Adam and his posterity, by God's institution and the law of creation, Rom.5:12, etc.
And the Holy Ghost, by representing the union that is between Christ and believers by such a variety of resemblances, in things agreeing only in the common or general notion of union, on various grounds, does sufficiently manifest that it is not of, nor can be reduced unto, any one kind of them.
And this will yet be made more evident by the consideration of the causes of it, and the grounds whereinto it is resolved. But whereas it would require much time and diligence to handle them at large, which the mention of them here, being occasional, will not admit, I shall only briefly refer unto the heads of them:
1. The first spring or cause of this union, and of all the other causes of it, lies in that eternal compact that was between the Father and the Son concerning the recovery and salvation of fallen mankind. Herein, among other things, as the effects thereof, the assumption of our nature (the foundation of this union) was designed. The nature and terms of this compact, counsel, and agreement, I have declared elsewhere; and therefore must not here again insist upon it. But the relation between Christ and the church, proceeding from hence, and so being an effect of infinite wisdom, in the counsel of the Father and Son, to be made effectual by the Holy Spirit, must be distinguished from all other unions or relations whatever.
2. The Lord Christ, as unto the nature which he was to assume, was hereon predestinated unto grace and glory. He was "proegnoosmenos",- -"foreordained," predestinated, "before the foundation of the world," 1 Pet.1:20; that is, he was so, as unto his office, so unto all the grace and glory required thereunto, and consequent thereon. All the grace and glory of the human nature of Christ was an effect of free divine preordination. God chose it from all eternity unto a participation of all which it received in time. Neither can any other cause of the glorious exaltation of that portion of our nature be assigned.
3. This grace and glory whereunto he was preordained was twofold:
(1.) That which was peculiar unto himself;
(2.) That which was to be communicated, by and through him, unto the church.
(1.) Of the first sort was the "charis henooseoos",--the grace of personal union; that single effect of divine wisdom (whereof there is no shadow nor resemblance in any other works of God, either of creation, providence, or grace), which his nature was filled withal: "Full of grace and truth." And all his personal glory, power, authority, and majesty as mediator, in his exaltation at the right hand of God, which is expressive of them all, do belong hereunto. These things were peculiar unto him, and all of them effects of his eternal predestination. But,
(2.) He was not thus predestinated absolutely, but also with respect unto that grace and glory which in him and by him was to be communicated unto the church.