First, He encourages, "Do not fear any of those things which you are about to suffer." He does not say He will take away the suffering, tacitly acknowledging that they will suffer. He is admonishing them to reorient their focus so that they fear Him rather than their circumstances. Revelation 21:8 says that the fearful and the unbelieving will go into the Lake of Fire, and this happens because they fear the wrong things. Thus, they have no part with God.
In many ways, what Revelation 2:10 describes is entirely foreign to us, yet many passages warn us that God's people will face tribulation. Peter writes, "Do not think it strange concerning the fiery trial which is to try you, as though some strange thing happened to you" (I Peter 4:12). We are so unaccustomed to persecution that we do indeed think it strange, but Paul tells Timothy, "All who desire to live godly in Christ Jesus will suffer persecution" (II Timothy 3:12).
Jesus warns us that we will be hated by all for His name's sake (Matthew 10:22), even delivered up to tribulation and death (Matthew 24:9). He prophesies that the time will come when whoever kills God's people will think he does God a service (John 16:2). John 16:33 is both cautionary and encouraging: "These things I have spoken to you, that in Me you may have peace. In the world you will have tribulation; but be of good cheer, I have overcome the world."
In Revelation 2:10, Jesus says that the Devil is about to throw some of them into prison to test them. A test perpetrated by Satan may not make much sense to men. It may not be over anything as dramatic as keeping the Sabbath or holy days or refusing the Mark of the Beast. It could simply be that, because the society has become so litigious and the civil law so overbearing, these saints become entangled without actually having done anything wrong. Nevertheless, as a test of their faith, God will allow Satan to jail them, for whatever reason—legitimate or not. God does this so that He and they know where their convictions stand—to see if they will compromise to ease their captivity, to see if they will remain faithful to God and His truth, and to see if they will trust Him even in tough times. It is during tumultuous times like the present that a person's character is revealed.
However, God is also merciful, telling Smyrna that its tribulation will be of limited duration. The church there can expect persecution and tribulation, but God has set limits on it, just as He did for Job (Job 2:6). He will not allow His saints to be tempted—proved, tried—beyond what they can bear (I Corinthians 10:13).
"Be faithful until death, and I will give you the crown of life," He says to conclude Revelation 2:10. Because this follows right on the heels of the Devil throwing some of them into prison, it almost sounds as if they will be in prison for ten days and then die, but it need not mean this at all. His exhortation to be faithful until death is universal, not just applicable for those thrown into prison. Whether we, like the apostle John, are allowed to die a natural death at an advanced age or, like Stephen, suffer martyrdom shortly after conversion, the command is the same: We must be faithful to our last breath. We cannot rest on the fact that we were faithful last year or last decade. Our faithfulness should be strong right to the finish line.
If we maintain our faithfulness, Christ gives us a crown of life. He similarly admonishes the church at Philadelphia to "hold fast what you have, that no one may take your crown" (Revelation 3:11). Paul calls it an "imperishable crown" (I Corinthians 9:25) and a "crown of righteousness" given "to all those who have loved and yearned for and welcomed His appearing (His return)" (II Timothy 4:8, Amplified Bible). James adds, "Blessed is the man who remains steadfast under trial, for when he has stood the test he will receive the crown of life, which God has promised to those who love him" (James 1:12, English Standard Version).