This point made about the witness of men is a general point, for John is not suggesting that we may receive the witness of the apostles to the gospel and yet reject the witness of God. The witness of the Spirit, the water and the blood is the witness of God.
If we see our slowness to trust him, how can we increase our trust? By reflecting on his integrity, on the fact that he has never broken his word, and that he will never do so. A man who hesitates to cross a bridge may reassure himself by considering the understanding of the one who designed it, the skill of those who built it, the number of those who have crossed it safely, and what great weight that bridge has already been able to bear. The change that is needed is in his heart, not in the bridge. We must reflect on the wisdom of God’s plan, the perfect way the Son of God has executed it, the abundance of the assurance given to those who come for pardon and life.