In Christ
Repetitio mater studiorum est. "Repetition is the mother of all learning." The Apostle Paul understood this. Under the inspiration and superintendence of the Holy Spirit, Paul constantly repeated the foundational truths of biblical doctrine, and he did so not only within each of his epistles but sometimes within the same sentence. The clearest example of this is found in Paul's epistle to the Ephesians. As he unfolds the glorious mystery of our salvation, Paul reiterates the phrase "in Christ" or "in Him" continually throughout the first chapter, and nearly ten times in verse 3–14, which is one long sentence in the original language. Several years ago as I preached through Ephesians chapter 1, I explained to our congregation that if they were only to remember one truth from our study of Ephesians, that it should be the phrase "in Christ," which is a shorthand way of remembering one of the most foundational aspects of salvation—our union with Christ.
The believer's union with Christ has long been a neglected doctrine in many churches, yet it is a central doctrine in Scripture. God's Word teaches us that we are chosen in Christ before the foundation of the world and that we are united to Christ by God's justifying grace alone through our faith alone because of the atoning death of Christ alone (John 15:4–7; 1 Cor. 15:22; 2 Cor. 12:2; Gal. 3:28; Eph. 1:4, 2:10; Phil. 3:9; 1 Thess. 4:16; 1 John 4:13). The nature of this union is not only that we are in Christ but that He is in us (John 6:56; Rom. 8:10; 2 Cor. 13:5; Gal. 2:20; Eph. 3:17; Col. 1:27). The theological implications of our union with Christ are astounding, and it is Christ Jesus Himself who taught us what they are. In John 15, Jesus said: "I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing" (v. 5). At the root of our sanctification is our union with Christ. As branches, we bear fruit precisely because we are united to Christ the vine, and we are connected to the vine because of the work of God the Father, who is "the vinedresser" (15:1). Moreover, in His high-priestly prayer, Jesus expressed the profound union He has with believers, saying, "I in them and you in me, that they may become perfectly one, so that the world may know that you sent me and loved them even as you loved me" (17:23). In this glorious prayer, Jesus reveals the absolute majesty of this doctrine when He expresses that our union with Him—the eternal Logos, the Son of God, the second person of the Godhead, God with us—has the direct implication that, in Christ, the Father loves us as He loves His only begotten Son. And since we are united to Christ, we are united with Him in His death, and, therefore, we shall also be united with Him in His resurrection (Rom. 6:5).