We are all, by nature of our physical birth, born into the family of Adam. But how does one get "in Christ?" The answer is found by comparing Matthew 1:18-25 and Luke 1:31-35 with John 3:1-21. Gabriel, the angel of the Lord, first told Mary, and later told Joseph, that the Holy Child she was carrying, Jesus, was conceived of the Holy Spirit. In other words, Jesus was of the family of Adam through Mary, and He was also born of the Spirit, literally, born of God (Acts 5:3-4). Jesus was born both physically and spiritually at the same time; He was both the "Son of Man" (Mt. 1:16), and the "Son of God" (Lk. 3:38)!
The Bible tells us that when we believe the Gospel, which declares that Jesus died for our sins, that He was buried, and that rose again from the dead, according to the Scriptures, we are born again with a spiritual birth; we have passed from being spiritually dead, to being spiritually alive (Jn. 3:1-21; 5:24; 1 Cor. 15:1-4; Eph. 2:1; Col. 2:13; 1 Pet. 3:18; etc.). Paul described our spiritual birth as being the result of our being baptized into Christ by the Holy Spirit (Rom. 6:3-4; Gal. 3:27).
In summary: "So then they that are in the flesh cannot please God. But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of His. And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness. But if the Spirit of Him that raised up Jesus from the dead dwell in you, He that raised up Christ from the dead shall also quicken your mortal bodies by His Spirit that dwelleth in you. Therefore, brethren, we are debtors, not to the flesh, to live after the flesh. For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live. For as many as are led by the Spirit of God, they are the sons of God" (Rom. 8:8-14).
How does one get to be in Christ? BELIEVE IN HIM!