"We are of God: he that knoweth God heareth us; he that is not of God heareth not us. Hereby know we the spirit of truth, and the spirit of error." (1 John 4:6)
Here we are given assurance that we will be able to tell the difference in people by the way they respond to the Word of God. The emphasis is on the believer's ability to discern a spirit (attitude or character) of truth or error among those to whom we witness.
Others disguise themselves as ministers of righteousness when they are really the ministers of Satan (2 Corinthians 11:14-15). How can we tell which is which?
The spirit of truth is relatively easy to discern. Those who hear the word (Mark 4:18-20) and receive the word with all readiness of mind (Acts 17:11) are of the truth (John 18:37). Such people come willingly to the light (John 3:21) and ask for a "reason of the hope that is in you" (1 Peter 3:15).
Some of these run among God's family and live "in error" (2 Peter 2:18). They can be fruitless trees and "raging waves . . . foaming out their own shame" (Jude 12-13), or like "tares" among the wheat that even the angels have trouble recognizing (Matthew 13:38-40). These won't listen to truth.
Our job is to be ready to give the answer to the one, and to reject the other. HMM III