'Regarding' in the Bible

If we accept [as we do] the testimony of men [that is, if we are willing to take the sworn statements of fallible humans as evidence], the testimony of God is greater [far more authoritative]; for this is the testimony of God, that He has testified regarding His Son.

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.