We understand the word “mystery” to imply something which the mind of man never could have understood except by revelation. “Great is the mystery of godliness [or piety]: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory” (1 Tim. 3:16). All the vain systems of philosophy and idolatry prove that man by searching could not find out God. All was mystery as to God. This verse contains an outline of this marvelous revelation to us. The mystery is revealed in God manifest in flesh, in our very form and likeness. God is not to us a mystery that needs revealing. He was manifest before us in the man, the Holy One of God; justified; sealed by the Holy Spirit as the sinless One in the midst of all the sin and darkness. God was then seen, or made visible, to angels, not merely as the Jehovah of Israel, but God revealed and preached to the world — the Gentiles. Thus revealed, God was believed on in the world, and, as man, received up into glory. The whole character, too, of God is visibly seen in the Son. Let us, with holy reverence, remember that he that has seen the Son has seen the Father also.
Things New and Old, 26:196