Are you not aware that the Devil knows Scripture too? He quoted it to Jesus during His Temptation.
In the Bible, the apostle Paul always used the phrase "in Christ" to designate genuine Christians. He never once used the phrase to designate non-Christians.
Therefore if any man is in Christ, he is a new creature; the old things passed away; behold, new things have come (2 Corinthians 5:17 NASB, emphasis added).
Greet Prisca and Aquila, my fellow workers in Christ Jesus (Romans 16:3 nasb, emphasis added).
To the saints in Ephesus, the faithful in Christ Jesus (Ephesians 1:1, emphasis added).
And I was still unknown by sight to the churches of Judea which were in Christ (Galatians 1:22 nasb, emphasis added).
When these people are designated to be "in Christ," this is an important designation to the apostle Paul. It is so important that he leaves no ambiguity whatever about how a person gets to be "in Christ." He goes to great lengths to show that it is not by sacraments or by works of any kind. It is not by accepting simultaneously two different paths of salvation. It is only by faith in Jesus Christ.
For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord (Romans 6:23 nasb).
He saved us, not because of righteous things we had done, but because of his mercy (Titus 3:5).
Therefore, my brothers, I want you to know that through Jesus the forgiveness of sins is proclaimed to you. Through him everyone who believes is justified from everything you could not be justified from by the law of Moses (Acts 13:38,39).
For by grace you have been saved through faith; and that not of yourselves, it is the gift of God; not as a result of works, that no one should boast. For we are His workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them (Ephesians 2:8-10 NASB).
So how can Roman Catholics be considered "in Christ" if they continue to deny the only basis for coming to be in Christ—justification by grace through faith alone?
I am very well aware of that.. So what's your point? Knowing scripture when you're in a battle defeats the devil, why do you think Jesus quoted scripture to him? Our battle is not against each other, but against the devil, which is a spiritual battle, against wickness in the heavenlies and high places. That is part of the spiritual armor given to us in Ephesions 6. The Word of God is the Sword of the Spirit.