21 if indeed you have heard Him and have been taught by Him, as the truth is in Jesus:
and have been taught by [in] Him,
The word “taught” focuses on the ongoing growth in Christ. We should translate the word “by” as “in.” The idea of “in Him” is to be in communion with Him. Our response to Christ is important to how we deal with sin. Christ is the sphere of the Christian life. He represents what we are. “In Him” means that Jesus is the framework within which teaching takes place. Jesus is both the subject and the object of what is taught.
as the truth is in Jesus:
Jesus is truth personified (Jn 14:6). When people accept Him as Savior, they line themselves up with truth itself. They need to be taught in accordance with the truth that is in Jesus. He is the norm for truth.
1 Jn 5: 20 And we know that the Son of God has come and has given us an understanding, that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life.
Truth apart from Christ is empty.
Truth takes on a different character when we know Christ personally. Truth apart from Christ is barren. The idea that Jesus paid for our sins personally momentously changes our relationship to Him. There is a radical difference after a person comes to Christ. Truth in Christ will set us free from the shroud of illusion.
From a biblical viewpoint truth can be known. There is no certainty without the God who reveals Himself. We can have finite certainty but not certainty about anything eternal. We can know “the truth as is in Jesus.” Greek philosophers in Ephesus and other parts of the Greco-Roman world knew only the finite world.
People of our day believe only what is apparently true for them. Someone else’s truth may be entirely different. The upshot of this thinking is the stance that there is no truth. The Bible presents the God who reveals Himself, the essence of absolute truth.
John 8: 12 Then Jesus spoke to them again, saying, “I am the light of the world. He who follows Me shall not walk in darkness, but have the light of life.”