Adam Clarke on 3 John 1:11

Follow not that which is evil - Μμ μιμου το κακον· Do not imitate that wicked man, i.e., the conduct of Diotrephes; be merciful, loving, and kind. For whatever profession any man may make, it will ever appear that he who doeth good is of God - he alone is the person who uses rightly the grace received from God, and he alone shall enjoy the Divine approbation;

While he that doeth evil - He who is unfeeling, unmerciful, unkind, hath not seen God - has no proper knowledge of that God whose Name is mercy, and whose Nature is love.