To remain in someone means to continue to have fellowship with him. See how you translated "remains in God" in [1 John 2:6](../02/06.md). Alternate translation: "continues to have fellowship with him" or "stays joined with him" (See: [[rc://en/ta/man/jit/figs-metaphor]])
John uses the words "seen" and "known" to say that the person who sins has never met Christ in a spiritual sense. A person behaving according to his sinful nature cannot know Christ. Alternate translation: "No one ... has ever truly believed in him" (See: [[rc://en/ta/man/jit/figs-doublet]])