Yes. You are "in the faith" if Jesus Christ (Messiah) GOD is in you. Period. Quite simple really.
I thought the enemy primarily uses lies, falsehood and deceit. Yet:
2 Corinthians 13:8 For we are not able to do anything against the truth, but rather only for the truth.
Where do you find Biblical support that the enemy uses the doubts of qualified people to "lead people away"?
Jesus (thru Holy Spirit inspired Matthew) provided the example of Peter's "doubt" for a reason:
Matthew 14:30-31 But when he saw the strong wind, he was afraid. And beginning to sink, he cried out, saying, “Lord, save me!” And immediately Jesus extended his hand and caught him and said to him, “You of little faith! Why did you doubt?”
Peter's fear led to his doubt and he began to sink. He had faith (was "in the faith"/in the boat, so-to-speak) but it was just a "little faith" at the time, evidently.
When Peter (an 'in the faith' person) feared death and began to doubt, how long did it take Jesus (who is powerful God) to extend His hand and catch him???
There's a reason God is strong when we are weak. And a reason He's there with us.
2 Corinthians 13:4 For indeed, he was crucified because of weakness, but he lives because of the power of God. For we also are weak in him, but we will live together with him because of the power of God toward you.
Are there qualified, in-the-faith people (that is having Jesus Christ in them, whether they recognize it or not, weak faith or strong faith) that will NOT live together with Him, in your opinion?