Jesus told us that many people who are convinced they are Christians, who called Him Lord, who even did miracles in His name, would be rejected by Him at the day of judgment – because they serve God but not according to the will of God.
These people were obviously not in His name, even though they said they were acting in His name. They were obviously not yet their Lord of thought, word, and deed – even though they called him Lord. Many will say to Me in that day, ‘Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?’ And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’ (Matthew 7:22-23) Whoever commits sin also commits lawlessness, and sin is lawlessness. 1 John 3:4. Jesus replied,”I tell you the truth, everyone who sins is a slave to sin. (John 8:34)
When Christ, in union with us, controls our every thought, word, and deed – then we became part of Christ’s body of believers – the bride of Christ, His church, the church of God.
Each member of His body acts according to the head, Christ. Christ’s body cannot have each member acting independently; otherwise each foot will go according its own will, each hand, each eye will be out of control of the head, without singleness of purpose, without unity of spirit, without Christ in control. Each part of Christ’s body or church must be totally controlled by the head – just like the parts of our physical body are all controlled by our head. When we are filled with the fullness of God, then (and not until then) Christ is truly our Master, the Lord, and King in mind, word, and deed. Then and only then, we are in Christ.
John explains who is in Christ: whoever keeps his word [obeys the Lord's commands spoken to him], truly the love of God is perfected in him; by this we know that we are in him. 1 John 2:5. If we love one another, God dwells in us, and his love is perfected in us. 1 John 4:12.
To be in Christ is also explained by Paul. Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new. 2 Cor 5:17; and those who are in Christ Jesus, walk not after the flesh, but after the Spirit. Rom 8:1.
This is to manifest ourselves as a new creature; this means we must live not according to our own pleasure and desires, not according to what we think is right, like we are serving God in our own ways. We live according to His desires and pleasures, according to His statutes and commandments. We live a new way of life, walking in truth and in Spirit, His words.
Someone who truly believes, seeks and repents, eventually becomes pure and produces the fruit of the spirit, which is love, joy, peace, patience [long-suffering], kindness, goodness, faith, gentleness [or meekness], and self-control. Gal 5:22-23
Bring forth fruit that is consistent with repentance. Matt 3:8. When you bear (produce) much fruit, My Father is honored and glorified, and you show and prove yourselves to be true followers of Mine. John 15:8
If we really believe in Jesus, we will keep seeking him until we are pure. The cross of Christ is where we crucify our lusts and affections. Everyone who has this hope in him purifies himself, just as he is pure. 1 John 3:2-3. And anyone who does not carry his cross and follow me cannot be my disciple. Luke 14:27