let him hear what the Spirit says to the churches
See commentary on Revelation 2:7.
He who overcomes
See Who is the Overcomer?
The double-negative οὐ μὴ [ou mē] emphasizing the impossibility of being hurt by the second death. It is the strongest negative assertion about the future of which the Greek language is capable.1
Although members of the church would experience martyrdom (the first death), they would be assured of avoiding the dreaded second deaththe Lake of Fire. See Births, Deaths, and Resurrections. It is by way of the second birth, being born again, that a person moves from the category of the second resurrection to the first resurrection. Over those, the second death has no power (Rev. Rev. 20:6+). The promise to the overcomer is given to all those who come to faith in Jesus Christthey need not fear the second death, being cast into the Lake of Fire (Rev. Rev. 20:14+). No believer ever finds his way to the second death. But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death (Rev. Rev. 21:8+).