Revelation 2:10-11
10 Fear none of those things which thou shalt suffer: behold, the devil shall cast some of you into prison, that ye may be tried; and ye shall have tribulation ten days: be thou faithful unto death, and I will give thee a crown of life.
11 He that hath an ear, let him hear what the Spirit saith unto the churches; He that overcometh shall not be hurt of the second death.
The Grace New Testament Commentary - Revelation 2:11
2:11 Turning to an application for all "the churches," Jesus states that "He who overcomes shall not be hurt by the second death." Arminian commentators err in taking this verse to mean, "Believers who do not overcome shall be hurt by the second death." Reformed commentators also err by reading it this way: "All true believers are overcomers and therefore will not be hurt by the second death." Both views have Jesus offering escape from hell for faithful obedience to Him. Since this flatly contradicts so many passages regarding eternal security and justification by faith alone (e.g., John 5:24; 10:27-30), it is better to understand Jesus' words as a figure of speech called a litotes.
A litotes is an assertion that understates the reality being referred to. If a person, in speaking about a certain baseball player, says, "He's no Babe Ruth," what he is saying is that the player is not a great home run hitter. A Biblical example of litotes is Heb 6:10: "God is not unjust to forget your work and labor of love which you have shown toward His name," meaning God will definitely remember all your hard work.
So when Jesus says, "He who overcomes shall not be hurt by the second death," He is making an understatement. He is saying that the faithful Christian will be more than amply repaid for whatever sacrifice he may make for Christ's sake, and that his eternal experience will be so far beyond the reach of the second death that it cannot be imagined. The suffering believers in Smyrna could rest in this glorious promise given to them from the Lord Himself.