Here is the verse from Revelation in context:
Revelation 12:10-12 (English Standard Version)
10 And I heard a loud voice in heaven, saying, "Now the salvation and the power and the kingdom of our God and the authority of his Christ have come, for the accuser of our brothers[a] has been thrown down, who accuses them day and night before our God. 11 And they have conquered him by the blood of the Lamb and by the word of their testimony, for they loved not their lives even unto death. 12 Therefore, rejoice, O heavens and you who dwell in them! But woe to you, O earth and sea, for the devil has come down to you in great wrath, because he knows that his time is short!"
This scripture is very widely used to mean that Christians are encouraged to just go around say that God did this for me and God did that for me, but when examined this is not quite the case. The key to understanding the entire phrase is the word, testimony and its usage and word, and its usage. Both are implying that there is a specific testimony tied to the first part of the verse. By the blood of the Lamb is implicitly tied to the word of their testimony. By removing the fact that it is the blood of the lamb and the word of their testimony, a person can be misled to believing that any testimony is fine, when in fact it is not. The testimony is categorically implying to the blood of the Lamb. How so, read this verse, Romans 10:9 (English Standard Version)9 because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. What the testimony of the believer should always be tied into is the fact of our salvation is given to us. Also, scriptures like this: I Timothy 6:12 "Fight the good fight of the faith. Take hold of the eternal life to which you were called and about which you made the good confession in the presence of many witnesses."
According to scripture testimony is defined as: Testimony
(1.) Witness or evidence (2 Thess. 1:10).
(2.) The Scriptures, as the revelation of God's will (2 Kings 11:12; Ps. 19:7; 119:88; Isa. 8:16, 20).
(3.) The altar raised by the Gadites and Reubenites (Josh. 22:10).
Neither of these instances implies a witness that cannot be backed or assured by the word of God. All other supposed testimonies are not really testimonies. They are words of encouragement or words of aspiration. To truly testify of a matter, the bible says that it has to be establishable as in these examples:
But if he does not listen, take one or two others along with you, that every charge may be established by the evidence of two or three witnesses.
1 Corinthians 14:27
If any speak in a tongue, let there be only two or at most three, and each in turn, and let someone interpret.
1 Corinthians 14:29
Let two or three prophets speak, and let the others weigh what is said.
2 Corinthians 13:1
[ Final Warnings ] This is the third time I am coming to you. Every charge must be established by the evidence of two or three witnesses.
1 Timothy 5:19
Do not admit a charge against an elder except on the evidence of two or three witnesses.
Anyone who has set aside the law of Moses dies without mercy on the evidence of two or three witnesses.
These verses clearly convey the purpose of the witness or testimony. Each is not explicit, but must remain in context to have any power or meaning to be established. Jesus even said in John 5:31: "If I alone bear witness about myself, my testimony is not deemed true". If the Son of God, Jesus the Christ, had to have witnesses to His testimony, why then do we spend our time testifying of the shallow things. Christ has left us examples of proper reason to testify. We should follow the examples that He himself gave in the apostles:
But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth."
and you killed the Author of life, whom God raised from the dead. To this we are witnesses.
And we are witnesses to these things, and so is the Holy Spirit, whom God has given to those who obey him."
To this day I have had the help that comes from God, and so I stand here testifying both to small and great, saying nothing but what the prophets and Moses said would come to pass:
Now Moses was faithful in all God’s house as a servant, to testify to the things that were to be spoken later,
1 John 1:2
the life was made manifest, and we have seen it, and testify to it and proclaim to you the eternal life, which was with the Father and was made manifest to us—
1 John 4:14
And we have seen and testify that the Father has sent his Son to be the Savior of the world.
2 Corinthians 8:3
For they gave according to their means, as I can testify, and beyond their means, of their own accord,
Matthew Henry writes about this in his analogue: "A wild beast rises out of the sea, to whom the dragon gives his power."
The attempts of the dragon proved unsuccessful against the church, and fatal to his own interests. The seat of this war was in heaven; in the church of Christ, the kingdom of heaven on earth. The parties were Christ, the great Angel of the covenant, and his faithful followers; and Satan and his instruments. The strength of the church is in having the Lord Jesus for the Captain of their salvation. Pagan idolatry, which was the worship of devils, was cast out of the empire by the
spreading of Christianity. The salvation and strength of the church, are only to be ascribed to the King and Head of the church. The conquered enemy hates the presence of God, yet he is willing to appear there, to accuse the people of God. Let us take heed that we give him no cause to accuse us; and that, when we have sinned, we go before the Lord, condemn ourselves, and commit our cause to Christ as our Advocate. The servants of God overcame Satan by the blood of the Lamb, as the cause. By the
word of their testimony: the powerful preaching of the gospel is mighty, through God, to pull down strong holds. By their courage and patience in sufferings: they loved not their lives so well but they could lay them down in Christ's cause. These were the warriors and the weapons by which Christianity overthrew the power of pagan idolatry; and if Christians had continued to fight with these weapons, and such as these, their victories would have been more numerous and glorious, and the effects
more lasting. The redeemed overcame by a simple reliance on the blood of Christ, as the only ground of their hopes. In this we must be like them. We must not blend any thing else with this. (Rev 12:12-17)