Union between Christ and his people was planned already in eternity, in the sovereign pretemporal decision whereby God the Father selected us as his own. Christ himself was chosen to be our Savior before the creation of the world (1 Pet. 1:20); Ephesians 1:4 teaches us that when the Father chose Christ, he also chose us.
" ...the conversion of a sinner being not owing to a man's self determination, but to God's determination, and eternal election, which is absolute, and depending on the sovereign Will of God, and not on the free will of man; as is evident from what has been said : and it being very evident from the Scriptures, that the eternal election of saints to the faith and holiness, is also an election of them to eternal salvation; hence their appointment to salvation must also be absolute, and not depending on their contingent, self-determining Will."
Election is the unchangeable purpose of God, whereby, before the foundation of the world, He has out of mere grace, according to the sovereign good pleasure of His own will, chosen from the whole human race, which had fallen through their own fault from the primitive state of rectitude into sin and destruction, a certain number of persons to redemption in Christ, whom He from eternity appointed the Mediator and Head of the elect and the foundation of salvation. This elect number, though by nature neither better nor more deserving than others, but with them involved in one common misery, God has decreed to give to Christ to be saved by Him, and effectually to call and draw them to His communion by His Word and Spirit; to bestow upon them true faith, justification, and sanctification; and having powerfully preserved them in the fellowship of His son, finally to glorify them for the demonstration of His mercy, and for the praise of the riches of His glorious grace; as it is written "For he chose us in him before the creation of the world to be holy and blameless in his sight. In love he predestined us to be adopted as his sons through Jesus Christ, in accordance with his pleasure and will--to the praise of his glorious grace, which he has freely given us in the One he loves." (Eph 1:4-6). And elsewhere: "And those he predestined, he also called; those he called, he also justified; those he justified, he also glorified." (Rom 8:30).
Canons of Dort (1.7)