Union with Christ is about communion and connection with Christ. It is your identity in Him, to be His child and He is your God. It is the basis for all your beliefs and actions as a Christian. It is the heart of the Bible, the Christian message. It is to be born again into His family and live the Christian life. Union with Christ is where He makes you one with Him.
Union with Christ is not merely agreeing to the facts. Demons (and Satan) know who Jesus is. Whenever the unclean spirits saw Jesus, they fell down before Him and cried out, “You are the Son of God (Mark 3:11).” James teaches, “You believe that God is one; you do well. Even the demons believe—and shudder (James 2:19)!” It is not enough just to know the facts. This is not union with Christ.
Union with Christ is not getting your needs met, “Thank you very much and good day, see you later.” A great crowd heard all that Jesus was doing, how He had healed many. The people pressed in hard, but they were only there to be healed (Mark 3:7-12). Or the large crowd in John 6 who followed Him because they saw the signs that He was doing. Then He even did a miracle of feeding them all (more than 5,000 people) with five loaves and two fish! When He taught them who He was— the bread of life, the Son of Man from God the Father— they grumbled and did not believe. This is not union with Christ.
Union with Christ is to be a new creation in Him. You must be born again. It is to put Him on and take the old you off. It is to be chosen, elected, predestined, and adopted in Him. In Him you have redemption through His blood, the forgiveness of your sins, according to the riches of His grace (Eph 1:3-7). Do not rely on your good works or your own righteousness, on your background or all you’ve accomplished, but rely on the righteousness which comes through faith in Christ, the righteousness from God that depends on faith (Phil 3:8-11). It is to be a citizen of heaven, not of this world.
Spurgeon said, “There is no joy in this world like union with Christ. The more we can feel it, the happier we are.” John Owen said, “Union with Christ is our adoption, our justification, our sanctification, our fruitfulness, our perseverance, our resurrection, our glory.” Do you know Jesus? Are you born again and forgiven of your sins? Do you have union with Him? There is no sweeter place to be than in Christ.