9 We accept human testimony, but God’s testimony is greater because it is the testimony of God, which he has given about his Son.

Read 1 John 5:9 Using Other Translations

If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son.
If we receive the testimony of men, the testimony of God is greater, for this is the testimony of God that he has borne concerning his Son.
Since we believe human testimony, surely we can believe the greater testimony that comes from God. And God has testified about his Son.