'Witness' in the Bible

We [who were with Him in person] have seen and testify [as eye-witnesses] that the Father has sent the Son to be the Savior of the world.

This is He who came through water and blood [His baptism and death], Jesus Christ—not by the water only, but by the water and the blood. It is the [Holy] Spirit who testifies, because the Spirit is the truth. [He is the essence and origin of truth itself.]

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.