and exchanged the glory and majesty and excellence of the immortal God for an image [worthless idols] in the shape of mortal man and birds and four-footed animals and reptiles.

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.