“For we are members of His body, of His flesh, and of His bones.”- Ephesians 5:30

Paul here speaks only of true believers. Men who are quickened by divine grace and made alive unto God. Of them, he says, not by way of romance, nor of poetical exaggeration, but as an undisputed matter of fact, “We are members of His body, of His flesh, and of His bones.” That there is a true union between Christ and His people in no fiction or dream of a heated imagination. Sin separated us from God, and in undoing what sin has done, Christ joins us to Himself in a union more real than any other in the whole world.

This union is very near, and very dear, and very complete. We are so near to Christ, that we cannot be nearer; for we are one with Him. We are so dear to Christ, that we cannot be dearer. Consider how close and tender is the tie when it is true that Christ loved us and gave Himself for us. It is a union more intimate than any other which exists among men; for “Greater love hath no man than this, that a man lay down his life for his friends.” We were His enemies when Christ died for us, that He might save us, and make us so one with Himself, that from Him our life should be drawn, and that in Him our life should be hid. It is, then, a very near and dear union which Christ has established between Himself and His redeemed; and this union could not be more complete than it is.

Even the incarnation of Christ is not more wonderful than His living union with His people. It is a thing to be considered often; it is the wonder of the skies; and is chief among those things which “the angels desire to look into.” On the surface of this truth, you may not see much; but the longer you gaze, and the more the Holy Spirit assists you in your meditation, the more you will see in this wonderful sea of glass mingled with fire. My soul exults in the doctrine that Christ and His people are everlastingly one. ~ C.H. Spurgeon

https://www.blueletterbible.org/comm/spurgeon_charles/sermons/2244.cfm