Children of the devil are characterized by falsehood, but God's children by truth. The old characteristic of falsehood should one for all be laid aside by the believer, because truth must govern him in his new relations.
Falsehood takes many forms.
It can be the outright effort to deceive by telling an untruth. It can take the form of slightly twisting the truth to deceive. Any time a man speaks one thing, and either knows or thinks something else, he is speaking falsehood. This the believer should avoid. In the present case Paul condemns falsehood because of what it does to one's relation to others in the same union with Christ.