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Demetrius is borne witness to by all
This can be stated in active form. Alternate translation: "All who know Demetrius bear witness of him" or "Every believer who knows Demetrius speaks well of him" (See: rc://en/ta/man/translate/figs-activepassive)
Demetrius
This is probably a man whom John wants Gaius and the congregation to welcome when he comes to visit. (See: rc://en/ta/man/translate/translate-names)
by the truth itself
"the truth itself speaks well of him." Here "truth" is described as a person speaking. Alternate translation: "everyone who knows the truth knows he is a good person" (See: rc://en/ta/man/translate/figs-personification and rc://en/ta/man/translate/figs-explicit and rc://en/ta/man/translate/figs-rpronouns)
We also bear witness
What John is confirming is implied and can be made specific here. Alternate translation: "We also speak well of Demetrius" (See: rc://en/ta/man/translate/figs-ellipsis)