We are plainly taught in the word of God that as many as have believed are one with Christ: they are married to him, there is a conjugal union based upon mutual affection. The union is closer still, for there is a vital union between Christ and his saints. They are in him as the branches are in the vine; they are members of the body of which he is the head. They are one with Jesus in such a true and real sense that with him they died, with him they have been buried, with him they are risen, with him they are raised up together and made to sit together in heavenly places. There is an indissoluble union between Christ and all his people: “I in them and they in me.” Thus the union may be described:–Christ is in his people the hope of glory, and they are dead and their life is hid with Christ in God. This is a union of the most wonderful kind, which figures may faintly set forth, but which it were impossible for language completely to explain.
Oneness to Jesus is one of the fat things full of marrow. For if it be so, indeed, that we are one with Christ, then because he lives we must live also; because he was justified by his resurrection, we also are justified in him; because he is rewarded and for ever sits down at his Father’s right hand, we also have obtained the inheritance in him and by faith grasp it now, and enjoy its earnest. Oh, can it be that this aching head already has a right to a celestial crown! That this palpitating heart has a claim to the rest which remaineth for the people of God! That these weary feet have a title to tread the sacred halls of the New Jerusalem! It is so, for if we are one with Christ, then all he has belongs to us, and it is but a matter of time, and of gracious arrangement when we shall come into the full enjoyment thereof. Truly, in meditation upon this topic, we may each of us exclaim, “My soul shall be satisfied as with marrow and fatness, and my mouth shall praise thee with joyful lips.”
Charles Spurgeon, “Good cheer for Christmas,” The Sermons of Charles Spurgeon, Sermons 401-600 (Vol 3)