21 if indeed you have heard Him and have been taught by Him, as the truth is in Jesus:
Instead of alienation to the life of God, some people in Ephesus embraced Christ and became Christians with eternal life. This happened when they “learned” about, “heard,” and were “taught by” Christ. All this happened at one point when they became believers.
The “if” here is an assumed fact (first class condition). There is no doubt that Ephesian Christians had heard the truth about Christ.
Paul now made a certain statement in light of their having learned about Christ. The Christian can have certainty about what God reveals. People today claim that we can know nothing for sure; that is the “futility” of verse 17. Christians do not know everything, but they know something.
you have heard Him [emphatic]
“Heard” focuses on what happened when we were first exposed to Christ. The word “Him” is emphatic. The idea of having “heard Him” does not mean to have heard about Him. Christians heard Him, not simply about Him. The idea is more about responding to Him.
Christianity is Person-centered.
Christ is the subject of Christianity. Christianity is not the acceptance of wonderful religious ideas but the reception of the person of Jesus Himself. If we know Christ personally, we will deal with sin in our lives. He is holy and sin is incompatible with Him. Jesus often made the statement, “He who has ears to hear, let him hear.” The idea is to appropriate to our life what we know about Christ.
Jn 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.”