Is a person who acts like a Christian but is not in Christ actually a Christian?
No. No unbeliever is in Christ. A person may try to walk or behave like a Christian, but unless he is in Christ, he is not one. Being in Christ comes by faith and faith alone in the Gospel, which is an act of God.
Les explains in this context that although an unbeliever may try to walk like a Christian or behave like one, unless he is in Christ, he is not one. The requirement for being in Christ is faith and faith alone in the Gospel, and that is an act of God. The promise of no condemnation applies to those who are in Christ Jesus, but it does not apply to those who merely act like Christians while remaining outside of Christ.