1 John 2:3

The principle is that it is not enough to understand the theory; we must put it into practice. E.g., what makes an artist? Not merely learning the rules of perspective and mixture of colours, but actually putting one's hand to brush and canvas. First attempts may be unsuccessful, but skill comes by patient practice. Cf. Rembrandt's advice to his pupil Hoogstraten: “Try to put well in practice what you already know; and in doing so you will, in good time, discover the hidden things which you inquire about'. To know about Christ, to understand the doctrine of His person and work is mere theory; we get to know Him and to know that we know Him by practice of His precepts. γινώσκω (cognosco) is to οἶδα (scio) as γίνομαι (fio) to εἰμί (sum). ἐγνώκαμεν, cognovimus, “we have got to know,” i.e. “we know”. τηρεῖν, “keep a watchful eye upon”. Cf. Matthew 27:36 : καὶ καθήμενοι ἐτήρουν αὐτὸν ἐκεῖ.