Did Old Testament believers have the same 'in Christ' position as New Testament believers?
No, Old Testament believers did not have that kind of position. The context distinguishes them from Grace Age believers, who are in Christ and in the body. This liberty in Christ Jesus is a distinct New Testament position, not the same as Old Testament standing.
Les explains that the Old Testament believer did not have the same 'in Christ' position that Grace Age believers have. In the setting of Acts 15, Paul had been ministering to Gentiles at Antioch, where they were first called Christians. Jerusalem believers, including Peter and the rest, were upset that Paul claimed these Gentiles were saved without keeping the law of Judaism. False brethren came in secretly to spy out the liberty believers have in Christ Jesus, aiming to bring them into bondage—back under the law. Les points out that while Old Testament believers did not have that position, as Grace Age believers we are in Christ and in the body, and Paul and Barnabas were as well. Paul refused to give in to subjection for even an hour, preserving that freedom in Christ.