Question:
Is it possible for one who is in Christ to fall out of that?

Answer:
No, it is not possible to fall out of being “in Christ,” because the Lord says that if we have believed in Him we shall “never perish, neither shall any man pluck them out of My hand…and no man is able to pluck them out of My Father’s hand” (John 10:28,29). We are “kept by the power of God” (1 Peter 1:5). “The blood of Jesus Christ His Son cleanseth us from [every] sin” (1 John 1:7, see J. N. Darby translation note).

His blood makes us safe, and His Word makes us sure. If we disobey our earthly parents we lose the close fellowship we once enjoyed with them, but we still are their child. So with our heavenly Father: sin stops that sweet fellowship He wants to have with us, but we still are His child from the moment we believed on Him and His work on the cross.