This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel. (Ephesians 3:6, ESV)

Today’s big question: what is the mystery of Christ?

Paul refers to the mystery of Christ in Ephesians 3:4–5 that “has now been revealed by the Spirit to His holy apostles and prophets.” The first thing to note is that this was never a mystery to God, since He knows all things (1 John 3:20). The Lord planned Christ’s redemptive work “before the foundation of the world” (Ephesians 1:4), and has now “made known to us the mystery of His will, according to His good pleasure” (Ephesians 1:9). But what changed from the Old Testament to the New Testament, and why is there a mystery in the first place?

One of the key things that “changed” was that God’s plan of salvation through Christ was fulfilled. There were hints of God’s plan in Genesis 3:15, which took place in the Garden of Eden immediately after the Fall. Throughout the Old Testament, particularly through the prophet Isaiah, God revealed some of His plan, but it was primarily through the Jews.

What God did in the Old Testament was really a picture of what was to come through Jesus Christ. God showed the Jews that they could not keep the law, and they had to keep making sacrifices to cover their sins. Then in the gospels, the ultimate sacrifice (Jesus Christ) came to cover their sins once and for all.

What many Israelites obviously did not realize is that God’s plan was not just for the Jewish people, but for everyone, both Jew and Gentile, who receives Jesus Christ as their Savior. God’s plan throughout Scripture has always been the same, and since the Fall, that plan has been in motion. God reveals to us only what we can handle and understand; He is all-knowing, and we have not even begun to fully comprehend the mind of God (Isaiah 55:8–9). Now that Christ’s work on the Cross has been completed, and the mystery has been revealed to us through Paul (Ephesians 3:8–9), it is time to boldly proclaim the gospel to everyone—both Jew and Gentile (Romans 10:14–17). http://www.answersingenesis.org/articles/2010/08/09/mystery-of-christ