Luther’s monumental exposition of Galatians, in which lies probably his most crystallized thought on the doctrine of justification by faith, illuminates a lot of Luther’s thought on the person of Christ. Luther finds in Galatians a distinction between Christian righteousness and any other type of righteousness. The latter is the righteousness which comes from keeping the law, but the former is the righteousness which comes only from Christ. In other words, the believer sees Christ as the source of his righteousness, for “the righteousness of faith [is that] which God imputes to us through Christ without works…it is a merely passive righteousness.” Indeed, in taking hold of Christ and His benefits, “we do not perform but receive.” Thus, for Luther, Christ is the one who gives the believer His righteousness, not only washing the believer clean before God but also imputing to the believer all the righteousness which Christ earned. What is more, it is integral that Christ be fully divine, because if He is not, then this doctrine is “illegitimate, in fact, sacrilegious.” But He is in fact God, and He “is not a cruel master; He is the Propitiator for the sins of the whole world,” for He “is not a lawgiver; He is a Propitiator and a Savior.” For Luther, gazing upon Christ and His work “is our highest comfort, to clothe and wrap Christ this way in my sins, your sins, and the sins of the entire world, and in this way to behold Him bearing all our sins. When He is beheld this way, He easily removes all the fanatical opinions of our opponents about justification by works.” Christ’s work, for Luther, has both a particular and a universal element, for it is for the individual person, and yet at the same time it is for the world as well. The work of the person of Christ is for Luther the most comforting thing, for “By this fortunate exchange with us He took upon Himself our sinful person and granted us His innocent and victorious person.”
The work of Christ, in addition to saving the believer, also tells the believer a great deal about himself specifically and about human nature generally. Christ’s humiliation tells the believer that human nature is greatly impaired at the very least after the fall, for “If the natural powers are unimpaired, what need is there of Christ?” The Christian sees in Christ the dire state of his own soul in addition to the dire state of humanity, for nothing other than the Son of God Himself would suffice to redeem him and to bring him into communion with God. To be certain, the believer can only be “concealed and covered by the cleanness and purity of Christ, which we obtain by hearing the Word and by faith.” Thus, one must always look to Christ and who he is in Him, rather than looking to himself, for he will never be pure in himself but will always be purer than snow in Christ. In fact, one can be pure only in Christ, for “Whoever does not have Christ, will not be saved—whether it be Moses, pope, cardinal…for God has placed His grace solely in the only Son.”
This doctrine of Christ is important for Luther, as it is for the entire Christian tradition, because for one thing, it is impossible to know God if one does not know Christ. For Luther, the person of Christ is the one through whom God is truly known. He asks, “Whence comes the knowledge of the God of grace and truth?” and, answering his own question, he says that its source is none other than Christ: “It is given by the only-begotten Son of God.” This is the case because there is “no other doctor, teacher, or preacher who resides in the Godhead and is in the bosom of the Father” but Christ. Thus, just as Christ’s work on behalf of the believer is effective only because He is God, so the knowledge He gives of God is legitimate because He Himself is God, residing “in the bosom of the Father,” as it were.
That said, the doctrine of Christ also remains crucial for Luther because if Christ is not God, then the work He did on behalf of believers has no efficacy. On the other hand, if Christ is not man, what He did means nothing because He has to be man to redeem man. Thus, Luther is nothing but creedal when he says that “We also believe that Christ, our Savior, is the true Son of Mary and the only-begotten Son of God; and yet there are not two sons but only on Son of God the Father and of the Virgin Mary.” Christ must be two natures in one person, “otherwise no satisfaction could have been rendered for our sins, and nothing would come of our salvation.” If Christ lacks divinity, no believer is saved because “no man’s suffering has ever been able to overcome my sin and yours,” and if he were two individual persons (rather than one person with two natures), “He would not be able to sit at the right hand of God as merely a human being.” Indeed, Luther is zealous to note that Christ’s divine nature was certainly still present during his 33 years on earth, for “He retained His Godhead unaltered, when, according to His human nature, He came down to earth.” For Luther, then, both Christ's person and work are absolutely crucial to the believer's experience. This, among many other reasons, is why it's so important that Christ is Himself Very God from Very God.