This is the message [of God’s promised revelation] which we have heard from Him and now announce to you, that God is Light [He is holy, His message is truthful, He is perfect in righteousness], and in Him there is no darkness at all [no sin, no wickedness, no imperfection].

whoever says he lives in Christ [that is, whoever says he has accepted Him as God and Savior] ought [as a moral obligation] to walk and conduct himself just as He walked and conducted Himself.

The one who believes in the Son of God [who adheres to, trusts in, and relies confidently on Him as Savior] has the testimony within himself [because he can speak authoritatively about Christ from his own personal experience]. The one who does not believe God [in this way] has made Him [out to be] a liar, because he has not believed in the evidence that God has given regarding His Son.