Can an unbeliever be in Christ by acting like a Christian?

No. No unbeliever is in Christ. A person may try to walk like a Christian or behave like one, but unless he is actually in Christ, he is not one. Being in Christ comes by faith and faith alone in the Gospel, and that is an act of God.

Les explains this from Romans 8:1, following the dilemma of Romans 7: despite failures, there is now no condemnation for those in Christ Jesus. The requirement is being in Christ. He emphasizes that 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. That position comes by faith and faith alone in the Gospel, and it is an act of God. So outward behavior cannot substitute for actually being in Christ.

Romans 8:1

There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit.

How does someone get to be in Christ Jesus?

A person is placed in Christ Jesus by faith and faith alone in the Gospel, and that positioning is an act of God. No unbeliever is in Christ; regardless of outward behavior, only those who are in Christ by faith possess that standing, and they have no condemnation.