"If the readers are to have fellowship with the Father and with the Son (v.3), they must understand what makes this possible. They must know who God is in himself and, consequently, who they are in themselves as creatures of God. So the author first describes the moral character of God in terms of light (v. 5) and then goes on to deny three claims made by those who falsely boast of their knowledge and fellowship with God. The false positions are (1) moral behavior is a matter of indifference in one's relationship to God (v. 6); (2) immoral conduct does not issue in sin for one who knows God (v. 8); and (3) the knowledge of God removes sin as even a possibility in the life of the believer (v. 10). True 'tests' or evidence of fellowship with God or walking in the light are (1) fellowship with one another (v. 7), with subsequent cleansing by the blood of Christ; (2) confession of sin, (v. 9) which brings both forgiveness and cleansing; and (3) trusting that if we sin we have Jesus Christ as an advocate and sacrifice for our sins (2:2)."
Barker, p. 309.

"By contrast, to 'walk in darkness' (verse 6) is to hide from
God and to refuse to acknowledge what we know about
Him."
Idem, The Epistles . .  pp. 60-61.

"To know God was not merely to know Him as the
philosopher knows Him; it was to know Him as a friend
knows Him. In Hebrew the word to know is used of the
relationship between husband and of [sic] wife, and
especially of the sexual act, the most intimate of all
relationships (cp. Genesis 4:1)."
Barclay, p. 64.

"In other words, to 'know' God is not a matter of correct
thought-processes, but of a genuine spiritual relationship.
The knowledge of God, and fellowship with him, are
complementary aspects of Christian experience."
Smalley, p. 45.