Could this promise in Rev. 3:10 to be kept from this great test just be a reference to some immediate testing that the Philadelphian believers in the first century would face? No, because the promise refers to a world-wide testing, not one of the local periods of severe persecutions encountered in the first and second centuries. There actually were not empire-wide persecutions until the 3rd-4th centuries (under Decian and Diocletian).
I have also heard advocates of a post-tribulation view state that God is able to keep His people through such a serious trial. That is certainly true, and God’s people have suffered through the ages while He has remained faithful to them, even when He allowed them to suffer. Sometimes He has delivered them from suffering, but sometimes He hasn’t! But our text does NOT say that He will keep us through the great test, but that He will keep us from the great test. Also, the text is quite specific that He will keep us from the very time period of the Tribulation. Rev 3:10 uses the expression “hour of testing.” The Greek word for “hour” is ὥρα (hora), a word that refers to the very time, not just the experience, of the tribulation. See also its use this way in Rev 11:13; 14:7, 15; 17:12; 18:10, 17, 19.
How then will these saints be “kept” from that time? While the text of Revelation does not make that clear, it is best to point to another passage that speaks of both “the dead in Christ” and “we who remain” being caught up to be with the Lord in the air (1Thess 4:16-17). And since that will take place before “the day of the Lord” (1Thess 5:2), the parallel with the Revelation passage makes perfect sense. It is the same “Day of the Lord” that comes on the whole earth, according to Rev 3:10, from which believers will be spared.
Then who are those “saints” that are mentioned in Rev. 13:10 who must patiently endure this time of trial? They must be those who come to faith after the rapture – those 144,000 Jewish believers (Rev 7:1-8) and the great multitude of Gentile believers who “come out of the great tribulation” (Rev 7:9-17).
In summary, the promise of Christ to the Philadelphia church saints is a promise to all church saints. That promise is that He will separate believers from a severe future period of testing by rapturing them from the earth at His imminent coming before the period of testing begins.
Is this an issue to fight over? Absolutely not! But some serious study of Rev 3:10 can certainly help us to address the issue. Prophecy is not a club to fight with, but a tool to build with. When we realize that, we will see that this teaching results in a blessed hope (Tit 2:13) that encourages us not to quit (1Thess 4:18).
If you can find a copy, I highly recommend Kept from the Hour by Gerald Stanton for the best defense of this view. Another excellent treatment is Maranatha: Our Lord, Come! by Renald Showers.