John notes about how testification is from seeing and hearing. The phrase accurately describes both John and Jesus. John testifies upon seeing Jesus (back in John 1), while Jesus testifies from heaven. When one receives the testimony on Jesus (either from Jesus or John), one confirms the truthfulness of God.
However, at that time, no one accepted it yet. The "no one" could refer to the crowds who are getting baptized or the people raising the question to John, but it is likely a figurative language used to describe how few the entire Israelite population accepted Jesus at that time.