Because of what will be happening at the end time, "persevering" or "courageously enduring" without compromising will certainly be no small accomplishment. Yet Christ says that because some of His people have been keeping His command to persevere, He will keep them from the worst of it. They have already proved their faithfulness to Him; He knows where they stand, He sees their track record with Him, and He will not require them to experience everything that the rest of humanity will suffer.
In colleges and universities, some professors make the final exam at the end of a semester optional. This means that students take the final only if they need to bring their overall average up. But if a student already has an A from other tests and class work, the professor figures the student has already proved himself, and does not require him to take the final exam.
This approach is analogous to Revelation 3:10. If the Christian is already faithfully persevering and resisting the spiritual foes, God may not require that he endure the very hardest test to prove what is in his heart. He has already proved it consistently through the course of his life. However, if, like a stereotypical first-year college student, he has frittered away his time, becoming involved in matters having nothing to do with college, he will have to prove where he stands. The final exam in this case is the Great Tribulation and Day of the Lord, so it is in our best interest that we students demonstrate to the Teacher that we are serious before the end of the semester.