Second Timothy directs the conduct of the faithful when confusion has come into [the Christian profession]. There is a rule for the faithful one: “Let every one that nameth the name of Christ depart from iniquity.”
We must expect to find in a great house vessels to dishonor as well as vessels to honor, but again there is a rule for the faithful one. He must purify himself from the vessels to dishonor and “follow righteousness, faith, [love], peace, with them that call on the Lord out of a pure heart.”
It is a frightful principle to say that we cannot distinguish between the children of God and the people of the world besides it is not true.
It is a frightful principle, for it is said, “By this shall all... know that ye are My disciples, if ye have love one to another.” Now, if I cannot discern them, I cannot love them, and the testimony which God desires is lost.
In the next place, it is not true practically, for we enjoy brotherly fellowship, and every faithful Christian makes a difference between a child of God and one who is not.
What would become of family affection, if a father said to his children, “You cannot tell who are your brothers and who are not; you must associate with everybody, without any distinction whatever!”
J. N. Darby (from Letters, Vol. 3)