1 John 5:10

10 Whoever believes in the Son of God accepts this testimony. Whoever does not believe God has made him out to be a liar, because they have not believed the testimony God has given about his Son.

Read 1 John 5:10 Using Other Translations

He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son.
Whoever believes in the Son of God has the testimony in himself. Whoever does not believe God has made him a liar, because he has not believed in the testimony that God has borne concerning his Son.
All who believe in the Son of God know in their hearts that this testimony is true. Those who don’t believe this are actually calling God a liar because they don’t believe what God has testified about his Son.