And they overcame him (cf. the frequent references to those who overcome, and the promises made to them, Revelation 2:1-29.; 3.; Revelation 21:7, etc.). The reference "they" is to "our brethren," the accused ones of Revelation 12:10.
By the blood of the Lamb; because of the blood, etc. (Revised Version). That is, "the blood of the Lamb" is the ground or reason of their victory, not the instrument. So in Revelation 1:9, "1 John … was in the island called Patmos, because of the Word of God ( διὰ τὸν λόγον)" (cf.
Revelation 6:9). Winer agrees with this view of the present passage, against Ewald and De Wette. "The Lamb," who was seen "as it had been slain" (Revelation 5:6)—Christ. And by the word of their testimony; and on account of the word, etc.
The one phrase is the natural complement of the other. "The blood of the Lamb" would have been shed in vain without the testimony, the outcome of the faith of his followers; that testimony would have been impossible without the shedding of the blood.
And they loved not their lives unto the death; their life even unto death. That is, they valued not their life in this world, even to the extent of meeting death for the sake of giving their testimony.