Remember now God is Spirit (Gospel of John 4:24) and it is the Spirit that bears witness, because Spirit is truth. (1 John 5:6)
So it was the Spirit of God who spoke by the mouth of the prophets of old, the apostles and yes Jesus the Christ. (Acts 1:2), (Ephesians 3:1-7)
Also (Psalm 119:160) says: the entirety of Your word is truth. .....