A slave can not establish his/her identity. Only the master can. Why? The slave is not entitled to ownership.
Christ establishes our identity through Him.
"For the mind set on the flesh is death, but the mind set on the Spirit is life and peace, because the mind set on the flesh is hostile toward God; for it does not subject itself to the law of God, for it is not even able to do so, and those who are in the flesh cannot please God. However, you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. But if anyone does not have the Spirit of Christ, he does not belong to Him."
Romans 8:6-9 NASB