It has been said that justification by faith alone is the doctrine by which the church stands or falls. Likewise in his Institutes of the Christian Religion John Calvin similarly wrote that justification is “the main hinge on which religion turns” (Ford Lewis Battles translation, p.726). In other words, the doctrine of justification by faith alone is in some way the central doctrine of true Christianity.
Benjamin B. Warfield said something similar about another central Christian doctine – the doctrine of the two natures (God and man) in the one person of Christ. He writes,
“[T]he doctrine of the two natures is only another way of stating the doctrine of the Incarnation; and the doctrine of the Incarnation is the hinge on which the Christian system turns. No Two Natures, no Incarnation; no Incarnation, no Christianity in any distinctive sense.” (The Works of Benjamin B. Warfield, Vol. III, p.259)
Warfield calls the doctrine of the incarnation of Christ “the hinge on which the Christian system turns.” Why? Because without it there really is no Redeemer, and so no gospel as well. Without the truth of the incarnation of Christ, you may still have a system of doctrine that goes by the name “Christian,” but it will not be truly Christian (to use Warfield’s phrase) “in any distinctive sense.”
In other words, it would be “Christian” in name only, and would then be essentially no different at its core from any other religion known to man, all of which (except for the biblical gospel alone) basically boil down to one form or another of salvation by works. You can either hold to a salvation by works (by self!), or a salvation by a Redeemer. And the only Redeemer (in order to actually be the Redeemer of sinners) must be both God and man in one person.
As the Westminster Shorter Catechism (Q.21) puts it,
“The only Redeemer of God’s elect is the Lord Jesus Christ, who, being the eternal Son of God, became man, and so was, and continues to be, God and man in two distinct natures, and one person, forever.”
The doctrine of the incarnation of Christ, which we celebrate every Christmas, really is “is the hinge on which the Christian system turns.” Without it, there is no real Christianity.