Compare Joh 15:4-10. To abide in Christ is more than to be in Him, since it represents a condition maintained by communion with God and by the habitual doing of His will. See on 1Jo 2:6.
Sinneth not
John does not teach that believers do not sin, but is speaking of a character, a habit. Throughout the Epistle he deals with the ideal reality of life in God, in which the love of God and sin exclude each other as light and darkness.
Seen - known
The vision of Christ and the appropriation of what is seen. Rev., correctly, knoweth.