RobH_en_tn/jhn/19/35.md

The one who saw this

This sentence gives background information to the story. John is telling readers that he was there and that we can trust what he has written. (See: rc://en/ta/man/translate/writing-background)

has testified, and his testimony is true

To "testify" means to tell about something that one has seen. Alternate translation: "has told the truth about what he has seen" (See: rc://en/ta/man/translate/figs-explicit)

so that you would also believe

Here "believe" means to put one's trust in Jesus. Alternate translation: "so that you will also put your trust in Jesus" (See: rc://en/ta/man/translate/figs-explicit)