This can be translated with an active verb. Alternate translation: "those whom others had killed" (See: [[rc://en/ta/man/translate/figs-activepassive]])
Here "Word of God" is a metonym for the message from God and "held" is a metaphor. Possible meanings are 1) holding the testimony refers to believing God's word and testimony. Alternate translation: "because of the teachings of scripture and what they taught about Jesus Christ" or "because they believed the word of God, which is his testimony" or 2) holding the testimony refers to testifying about the word of God. Alternate translation: "because they testified about the word of God" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonymy]])