Carson: “But in one crucial verse (Rev.12:11), John tells us how these believers overcome the devil.
(1) “They overcame him by the blood of the Lamb.” The preposition translated as by in the NIV should be rendered “on the ground of.” When all his accusations are brought before us — so many of them entirely justified, if we gauge things only by the quality of our faithfulness — Satan is silenced when we insist that our acceptance before God is grounded not in ourselves but in the death of Jesus Christ. “Who is he that condemns?” Paul exultantly asks. “Christ Jesus, who died — more than that, who was raised to life — is at the right hand of God and is also interceding for us” (Rom. 8:34). We neither have nor need another ground for our acquittal.
(2) “They overcame him by ….the word of their testimony.” This does not mean that they frequently gave their testimonies. It means, rather, that they constantly bore testimony to Jesus Christ; in short, they constantly proclaimed the Gospel.”