“I have not written to you because you do not know the truth, but because you know it, and that no lie is of the truth.”
I have not written to you because you do not know the truth,
The purpose of John’s writing was not to inform his readers of something new but to bring them to the point of applying truth they already know to experience.
but because you know it,
John’s readers knew the truth but allowed false teachers to sway their view of Christ. They knew they had the indwelling Holy Spirit to protect them from error (1 Jn 2:20) but did not allow their “anointing” to stop the infiltration of false doctrine. They are far more culpable than non-Christians because of satanic blindness. They needed to apply what they knew.
2 Pe 1:12, “For this reason I will not be negligent to remind you always of these things, though you know and are established in the present truth.”
and that no lie is of the truth
The “lie” in this context is the teaching of the antichrists (1 Jn 2:18). These false teachers undermined the person and work of Christ. They denied that Jesus was the Messiah (1 Jn 2:22).
The “truth” is biblical revelation. God’s revelation is always consistent with the One who gave it. God can never contradict Himself. God is truth, so by His very nature, He cannot lie.
The word “of” means source. No lie has its source in the truth because truth and falsehood are radically irreconcilable. There is no neutral ground between truth and a lie. Truth can be nothing but the truth. It is always absolute.
Jn 8:44, “You are of your father the devil, and the desires of your father you want to do. He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he speaks a lie, he speaks from his own resources, for he is a liar and the father of it.”
Truth and error always stand in antagonism to each other.
Truth and error are mutually exclusive in God’s economy. Each time we fail to apply the principles of God’s Word to experience, we make God a liar. He is not a liar, but we make Him appear as a liar. Whenever we allow legalism, asceticism, or an outright sin to come into our lives, we make God out to be a liar. We especially make Him out to be a liar when we deny the truth that is in Jesus.
1 Jn 1:5, “This is the message which we have heard from Him and declare to you, that God is light and in Him is no darkness at all.”
There is no possible reconciliation between truth and error for the Bible-believing Christian. There are no gray areas when it comes to the truth of Jesus Christ or His work. There are no gray areas when it comes to absolutes that flow from that truth. The Christian has the anointing from the Holy Spirit to help him discriminate between right and wrong.
The believer in absolutes does not blend truth and error. Some areas are gray in the Christian life, but these are not in the absolute category of right and wrong. Christians cannot hold to relative ethics and still believe in the Bible. “It is okay to have sex with my girlfriend if I love her.” This kind of love justifies sin. Biblically, this is the rationalization of sin. It is a lie and a falsehood.
Ti 1:2, “…in hope of eternal life which God, who cannot lie, promised before time began…”
The believer has the Spirit of God in his heart and the Word of God in his mind to protect him from error. Every Christian has the ability to understand divine revelation because he has the indwelling Holy Spirit in him. The Holy Spirit will unfold and illuminate God’s Word if we are willing to receive it.
Non-Christians cannot understand divine truth, for all they have is a soul but not the spirit that comes from the new birth in Christ.
1 Co 2:14, “But the natural man [the man who is nothing but mind, emotion and will; he does not have the life of God in him] does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned.”