The union of a believer with Christ is so intimate, so unalterable, so rich in privilege, so powerful in influence, that it cannot be fully represented by any description or similitude taken from earthly things. The mind, like the sight, is incapable of apprehending a great object, without viewing it on different sides.
To help our weakness, the nature of this union is illustrated, in the Scripture, by four comparisons, each throwing additional light on the subject, yet all falling short of the thing signified.
In our natural state, we are κλυδωνιζομενοι και περιφερομενοι, driven and tossed about, by the changing winds of opinion, and the waves of trouble, which hourly disturb and threaten us upon the uncertain sea of human life. But faith, uniting us to Christ, fixes us upon a sure foundation, the Rock of Ages, where we stand immovable, though storms and floods unite their force against us.
By nature we are separated from the divine life, as branches broken off, withered and fruitless. But grace, through faith, unites us to Christ the living Vine, from whom, as the root of all fulness, a constant supply of sap and influence is derived into each of his mystical branches, enabling them to bring forth fruit unto God, and to persevere and abound therein.
By nature we are συγμτοι και μισουντες, hateful and abominable in the sight of a holy God, and full of enmity and hatred towards each other. By faith, uniting us to Christ, we have fellowship with the Father and the Son, and joint communion among ourselves; even as the members of the same body have each of them union, communion, and sympathy, with the head, and with their fellow-members.
In our natural estate, we were cast out naked and destitute, without pity, and without help (Ezek. 16); but faith, uniting us to Christ, interests us in His righteousness, His riches, and His honours. Our Redeemer is our husband; our debts are paid, our settlements secured, and our names changed.
Thus the Lord Jesus, in declaring Himself the foundation, root, head, and husband, of His people, takes in all the ideas we can frame of an intimate, vital, and inseparable union. Yet all these fall short of truth; and He has given us one farther similitude, of which we can by no means form a just conception till we shall be brought to see Him as he is in His kingdom. John. 17:21: ‘That they all may be one, as Thou, Father, art in Me, and I in Thee; that they also may be one in us.’
Well may we say: What hath God wrought! How inviolable is the security, how inestimable the privilege, how inexpressible the happiness, of a believer! How greatly is he indebted to grace! He was once afar off, but he is brought nigh to God by the blood of Christ: he was once a child of wrath, but is now an heir of everlasting life. How strong then are his obligations to walk worthy of God, who has called him to His kingdom and glory!”
–John Newton, “Letter XXVII” in The Works of the John Newton, Volume 1 (London: Hamilton, Adams & Co., 1824), 322-323.