John Murray on union with Christ:
Union with Christ has its source in the election of God the Father before the foundation of the world and it has its fruition in the glorification of the sons of God.
The perspective of God's people is not narrow: it is broad and it is long. It is not confined to space and time: it has the expanse of eternity.
Its orbit has two foci, one the electing love of God the Father in the counsels of eternity, the other glorification with Christ in the manifestation of His glory. The former has no beginning, the latter has no end.