Did Old Testament believers have the position of being in Christ?
No. According to the context, the Old Testament believer did not have the position of being in Christ. That position belongs to Grace Age believers, who are in Christ and in the body. The context contrasts the Old Testament believer's position with that of Grace Age believers.
Les explains that the Old Testament believer did not have the same position as Grace Age believers. In the context, Paul is defending the liberty Gentiles have in Christ Jesus, being free from the law. The Jerusalem believers were trying to bring them into bondage by requiring law-keeping. Les points out that when a person believes the gospel, they are in Christ, in the body—something the Old Testament believer did not have. This contrast underscores the unique position of Grace Age believers.