οὐκ ἔγραψα ὑμῖν ὅτι οὐκ οἴδατε τὴν ἀλήθειαν ἀλλ’ ὅτι οἴδατε αὐτήν καὶ ὅτι πᾶν ψεῦδος ἐκ τῆς ἀληθείας οὐκ ἔστιν   τίς ἐστιν ὁ ψεύστης εἰ μὴ ὁ ἀρνούμενος ὅτι Ἰησοῦς οὐκ ἔστιν ὁ Χριστός οὗτός ἐστιν ὁ ἀντίχριστος ὁ ἀρνούμενος τὸν πατέρα καὶ τὸν υἱόν

I didn’t write to you because you’re oblivious to the truth - no! You’re well aware of it.  No matter how convincing, a lie isn’t truth.  Want to know who the liar is?  Whoever skews Jesus into their own image - an image far from the original anointed one.  They’re substitute-anointed - impostors and their imaginary Father and Son.