The believer’s spiritual union with Christ is a doctrine distilling great comfort, and is key to the doctrine of the perseverance of the saints. Richard Hooker (1554-1600) picks up on this in his A Discourse of Justification:
He that hath the Son, hath life, saith St. John, and he that hath not the Son of God, hath not life (1 Jn. 5:12). If therefore he which once hath the Son, may cease to have the Son, though it be for a moment, he ceaseth for that moment to have life. But the life of them which have the Son of God, is everlasting in the world to come (1 Jn. 5:13). But because as Christ being raised from the dead died no more, death hath no more power over him (Rom. 6:10, Cf. Hooker’s Sermon on the Perpetuity of Faith); so justified man being allied to God in Jesus Christ our Lord, doth as necessarily from that time forward always live, as Christ, by whom he hath life, liveth always (Jn. 14:19)…
For as long as that abideth in us, which animateth, quickeneth, and giveth life, so long we live, and we know that the cause of our faith abideth in us for ever. If Christ, the fountain of life, may flit and leave the habitation, where once he dwelleth, what shall become of his promise, I am with you to the world’s end? If the seed of God, which containeth Christ, may be first conceived and then cast out; how doth St. Peter term it immortal (1 Pet. 1:23? How doth St. Peter affirm it abideth (1 Jn. 3:9)? If the Spirit, which is given to cherish and preserve the seed of life, may be given and taken away, how is it the earnest of our inheritance until redemption (Eph. 1:14)? How doth it continue with us for ever (Jn. 14:14)? If therefore the man which is once just by faith, shall live by faith, and live for ever, it followeth, that he which once doth believe the foundation, must needs believe the foundation for ever. If he believe it for ever, how can he ever directly deny it? Faith holding the direct affirmation; the direct negation, so long as faith continueth, is excluded.
– Richard Hooker (1554-1600), The Works of that Learned and Judicious Divine Mr. Richard Hooker (1793 edition), 3:462-463.