Even if we accept human testimony, the testimony of God is greater. For this is the testimony that God has given about His Son.
9 If we receive the witness, or testimony, of men He proves, reasoning from the less to the greater, how ungrateful men are when they reject Christ, who has been approved, as he has related, by God; for if in worldly affairs we stand to the words of men, who may lie and deceive, how unreasonable it is that God should have less credit given...
The faith of the Christian believer (or the believer in Christ) being thus mighty and victorious, it had need to be well founded, to be furnished with unquestionable celestial evidence concerning the divine mission, authority, and office of the Lord Jesus; and it is so; he brings his credentials along with him, and he brings them in a way by which he came and in the witness that attends him.
Commenting on 1 John 5:6-9
If we receive the witness of men,.... The witness of a sufficient number of credible men, of men of good character and report, is always admitted in any case, and in any court of judicature; it was allowed of in the law of Moses; everything was proved and established hereby; upon this men were justified or condemned, cognizance was taken of men's sins, and punishment...