do you believe this person is a believer?
If someone willingly believes false doctrines are they a believer...
Well... the devil is a believer.
Satan believes in Jesus and spiritual things we do not understand.
Being a believer does not automatically make you saved.
The 10 virgins are a good parable to show this
All ten believed, all ten were waiting for the coming of the bridegroom.
Waiting and believing in Jesus.
Mat 25:1-13 1 Then shall the kingdom of heaven be likened unto ten virgins, which took their lamps, and went forth to meet the bridegroom. 2 And five of them were wise, and five were foolish. 3 They that were foolish took their lamps, and took no oil with them: ....... and they that were ready went in with him to the marriage: and the door was shut. 11 Afterward came also the other virgins, saying, Lord, Lord, open to us. 12 But he answered and said, Verily I say unto you, I know you not. 13 Watch therefore, for ye know neither the day nor the hour wherein the Son of man cometh.
2Th 2:10-12
10 And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. 11 And for this cause God shall send them strong delusion, that they should believe a lie: 12 That they all might be damned who believed not the truth, but had pleasure in unrighteousness.