A servant’s life is “in” his master. A servant has laid aside his life to do the will of his master.
Jesus did the will of His Father. He said “I only do the things I see the Father do” and “I only say the things I hear the Father say”. He also said “I am in the Father”.
Now Jesus is our master, and as we lay aside our lives to do His will, we find our life “in Him”. And we also find that as we live in Him, so He also lives in us. Jesus said “I am in the Father and the Father is in Me” and “it is the Father living in Me who does the works”.
Speaking of the life of a believer, Peter says “…so that he can no longer spend the rest of his natural life living by human appetites and desires, but he lives for the will of God.” (1 Pet 4:2)
“I have been crucified with Christ, and I no longer live, but Christ lives in me. The life I live in the body I live by faith in the Son of God who loved me and gave Himself for me.” (Gal 2:20)
Is not the confession unto salvation “Jesus is Lord”?