Zechariah 12:1 - Thus says the Lord, Who stretches out the heavens and lays the foundation of the earth and forms the spirit of man within him.
God is Spirit, and Zechariah 12 tells us that God formed the spirit of man within him. The human spirit is what makes relationship and fellowship with God possible at all. Through his spirit, man can walk in step with God. In the Spirit he is one with God.
Through his spirit he knows of God and receives everything God wants to communicate to Him. It is in Man's spirit that Christ takes up residence.
He who puts on Christ is one spirit with Him.
1 Corinthians 6:17 - But he who is united to the Lord becomes one spirit with him.