Galatians 3:26-27
26For ye are all the children of God by faith in Christ Jesus.
27For as many of you as have been baptized into Christ have put on Christ.
Faith is a spiritual thing that God can detect. If you have faith in Christ Jesus the Bible says you're spiritually baptized into Jesus. God takes care of the rest.
1 Corinthians 3:1
1And I, brethren, could not speak unto you as unto spiritual, but as unto carnal,
even as unto babes in Christ.
Just to make sure we eliminate any possibility that we can earn our ticket into the body of Christ by being "good enough" at least one example is in order.
1 Cor. 3:1 says that even worldly Christians are still in Christ. Paul refers to worldly, unspiritual, Christians as babies who are in Christ. The only qualifier to be in Christ is having faith. The Holy Spirit will gradually transform those who are baptized into Christ from one glory to another by progressive sanctification. (2 Cor. 3:18, Hebrews 10:14)