In order to make clear who is the subject of this verse, it may be well to imitate Good News Translation: “Our fellow servants (or, Our brothers and sisters) have defeated him (the Devil)….”

They have conquered him: for the verb see 2.7. Here the defeat of the Devil is attributed to the faithful believers, the followers of Jesus Christ.

By the blood of the Lamb: see 1.5; 5.9; 7.14. By means of the sacrificial death of Jesus Christ, believers are able to defeat Satan.

By the word of their testimony: this, of course, is the gospel; “by means of the message they announced” or “by means of the truth they proclaimed” (see 1.2, 9). In certain languages it will be well to reorder these clauses and say “The blood of the Lamb and the true message that they proclaimed let them (or, caused them to) have the victory over the Devil.” In other languages the word of their testimony has been translated as “the word of God that they proclaim.”

For they loved not their lives even unto death: this is not too clear in English; New Revised Standard Version is better: “for they did not cling to life even in the face of death” (see Mark 8.35; John 12.25). The force of the statement is that they were willing to pay the price of martyrdom in order to be faithful to Jesus Christ. It may be that Revised Standard Version for (also Translator’s New Testament; New Jerusalem Bible “because”) correctly interprets the Greek conjunction, which is normally understood to mean “and.” But it is difficult to understand how for relates to what precedes. It seems better to interpret as Good News Translation has done (also Revised English Bible, New American Bible, Revised, Biblia Dios Habla Hoy, Traduction œcuménique de la Bible), as an additional reason, or as the underlying attitude that enabled them to defeat Satan. One may also say “and they were even willing to die if necessary,” “holding their lives lightly, they were prepared to die,” or “They said, ‘Let them kill us (if need be).’ They did not fear death.”

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on The Revelation to John. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .