Don’t let head-knowledge deceive you. NOBODY knows the truth who does not have fellowship with the upright in body of Christ. Jesus is God’s truth, and without fellowship with him and those who walk with him in light, there is no knowledge of the truth.
I could name a number of people, now backslidden, who thought they knew the truth because their heads could understand the doctrine.
Fellowship is the whole thing! Without it, there is no true life – none of the life Jesus suffered and died for.