This can be stated in active form. Alternate translation: "All who know Demetrius testify that he is a good man" or "Every believer who knows Demetrius speaks well of him" (See: [[rc://en/ta/man/translate/figs-activepassive]])
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]])
"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]])
What John is confirming is implied and can be made specific here. Alternate translation: "We also testify about Demetrius" or "We also speak well of Demetrius" (See: [[rc://en/ta/man/translate/figs-ellipsis]])