25 The ὅτι (hoti) clause in 5:11 is epexegetical (explanatory) to the phrase καὶ αὕτη ἐστίν (kai hautē estin) at the beginning of the verse and gives the content of the testimony for the first time: “And this is the testimony: that God has given us eternal life, and this life is in his Son.”
26 In understanding how “God’s testimony” (added to the three witnesses of 5:8) can consist of eternal life it is important to remember the debate between the author and the opponents. It is not the reality of eternal life (whether it exists at all or not) that is being debated here, but rather which side in the debate (the author and his readers or the opponents) possesses it (this is a key point). The letter began with a testimony that “the eternal life” has been revealed (1:2), and it is consummated here with the reception or acknowledgment of that eternal life as the final testimony. This testimony (which is God’s testimony) consists in eternal life itself, which the author and the readers possess, but the opponents do not. This, for the author, constitutes the final apologetic in his case against the opponents.

In other versions

And this is the testimony, that God gave us eternal life, and this life is in his Son.

And this is the testimony: God has given us eternal life, and this life is in his Son.

And this is the testimony: God has given us eternal life, and this life is in his Son.

And this is what God has testified: He has given us eternal life, and this life is in his Son.

And this is the testimony: that God has given us eternal life, and this life is in his Son.

And this is the record, that God hath given to us eternal life, and this life is in his Son.

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