To redeem those under the law, that we might receive adoption to sonship. (NIV)
To purchase the freedom of (to ransom, to redeem, to atone for) those who were subject to the Law, that we might be adopted and have sonship conferred upon us [and be recognized as God's sons]. (AMP)
To redeem them that were under the law, that we might receive the adoption of sons. (KJV)