“1. In Christ all the riches of God and the knowledge of Him are laid up, as the treasury and subject of them; and so discovered and communicated to us objectively in the knowledge of Him. Thus, in Colossians 2:2-3, the apostle further explains it.
For having termed the gospel the mystery of God and of Christ, he adds, ‘In whom are hid all the treasures of wisdom and knowledge;’ not only to be revealed by Him, or subjectively known in and by Himself; but (which is the proper scope of the apostle) objectively set forth, and contained in Him alone, and in the knowledge of Him made known to us.
2. In Christ shines ‘the glory of God’ (2 Cor. 4:6, ‘The light of the knowledge of the glory of God in the face of Jesus Christ’), as the lively image of all His features and perfections, and evidences of His inward counsels and affections.
That therefore which I here insist upon is, that Jesus Christ in all His glories is the great and eminent subject of the gospel, (Rev. 1:1).
3. It is the gospel of God (namely, as the author of it), but it is concerning His Son Jesus Christ our Lord. God had but one Son, and He made this gospel on purpose to honour Him, and set Him forth.
It is all, and every word of it, some way or other concerning Him, or about Him. God made it purposely to set His Son Christ forth to us; and in setting forth His Son, Himself also.
It is therefore termed (Colossians 2:2) ‘the mystery of God the Father, and of Christ,’ Christ, in that series of truths about Him held forth in the whole New Testament, is the sum of this newly revealed wisdom of God, (1 Cor. 1:24).”
–Thomas Goodwin, “A Discourse of the Glory of the Gospel,” The Works of Thomas Goodwin, Volume 4 (Grand Rapids, MI: Reformation Heritage, 1862/2021), 4: 263–264.