Martin Luther on John 16:3:
The true knowledge of God and of Christ has often been defined. It is not an idle, empty thought or dream, as reason is able to think of God and Christ on the basis of hearsay, and as it pictures Him and acts toward Him according to such thoughts of its own. No, it is the true and living faith, which understands the words of the Gospel and, in accordance with those words, knows Him and the Father’s will and heart. It knows that the Father sent Christ, His Son, to deliver the world from sin, God’s wrath, and eternal death through His blood and death. It tells man that Christ accomplished all this, gained forgiveness of sin and eternal life, and surely bestows this on all who believe in Him. Thus the knowledge of God and of Christ are bound together and are one knowledge. As has been repeatedly said, the Father is known solely in Christ and will not and cannot be attained and met or worshiped and called upon apart from this Mediator.
Therefore all depends on this article about Christ, and he who has this article has everything. In order to be able to abide by it, the Christians must be engaged in the most strenuous warfare and must fight constantly. Therefore Christ and the apostles have good reason to insist on it everywhere. Although the other doctrines are also based on Scripture – for example, Christ’s birth from a pure virgin – it does not stress them as much as it does this one. When St. Paul champions this doctrine of Christ’s birth, he does not even call the mother by name; nor does he mention the honor of the virgin; he states simply (Gal. 4:4) “born of woman.” But when he informs us that we receive grace and salvation, not by works and the Law but only through this Mediator, Christ, then he speaks exhaustively.
Source: https://www.puritanboard.com/threads/the-true-knowledge-of-god.91137/, Comment 1