The one who believes in the Son of God (theou | θεοῦ | gen sg masc) has the testimony in himself; the one who does not believe God (theō | θεῷ | dat sg masc) has made him out to be a liar, because he has not believed in the testimony that God (theos | θεός | nom sg masc) has borne concerning his Son.
Reference
1 John 5:10
Sequence
1056