Lit., the one knowing: he who is habitually and ever more clearly perceiving and recognizing God as his Christian life unfolds. The knowledge is regarded as progressive and not complete. Compare Phi 3:12, and He who is calling (ὁ καλῶν, 1Th 5:24) also ὁ ἀγαπῶν he that loves (1Jo 4:7).
Hereby (ἐκ τούτου)
Not the same as the common ἐν τούτῳ (1Jo 4:2). It occurs only here in the Epistle. Ἑν τούτῳ is in this: ἐκ τούτου from this. The former marks the residing or consisting of the essence or truth of a thing in something the apprehension of which conveys to us the essential nature of the thing itself. The latter marks the inference or deduction of the truth from something, as contrasted with its immediate perception in that something. Rev., by this.
The spirit of error (τὸ πνεῦμα τῆς πλάνης)
The phrase occurs nowhere else in the New Testament. Compare πνεύμασι πλάνοις misleading spirits, 1Ti 4:1.