When the Great Tribulation comes, how can people be saved? Is it still by faith or plain works?

You have hit on a very controversial topic among Independent Baptists. I have friends who vehemently defend both sides of this issue. Some affirm absolutely that salvation in the tribulation is by faith and works. Others affirm just as absolutely that salvation in the tribulation must be just as it is today--by faith alone. Both sides have their scriptural proofs and both sides have a tendency to accuse the other side of heresy.

Personally, I think it stinks to high heaven that groups of people who agree on 98% of doctrine castigate each other about the method of salvation in a prophetic time--a time we probably have a very faulty understanding of at best. I think it is a sign of the spiritual stagnation in America that we can waste time attacking each other on such an issue. They strive "about words to no profit, but to the subverting of the hearers" (1 Timothy 2:14And Adam was not deceived, but the woman being deceived was in the transgression.
See All...). Paul warned, "But shun profane and vain babblings: for they will increase unto more ungodliness" (1 Timothy 2:16
See All...). However, in the interest of answering your question, I will give you my best effort at this time.

The time of the tribulation is a time greatly different than any man has known. The New Testament believers will have been removed by the rapture and the devil will seem to have full sway through the antichrist and false prophet. There are things to deal with that never occurred before. For instance, the mark of the beast will keep a person from ever being saved. Revelation 14:9-10 [9] And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, [10] The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:
See All... states, "If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb." Evidently, those who take the mark of the beast will certainly be damned.

This and other various statements lead many to teach that salvation in the tribulation requires works. Some point to the judgment of the nations in Matthew 25:31-46 [31] When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory: [32] And before him shall be gathered all nations: and he shall separate them one from another, as a shepherd divideth his sheep from the goats: [33] And he shall set the sheep on his right hand, but the goats on the left. [34] Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world: [35] For I was an hungred, and ye gave me meat: I was thirsty, and ye gave me drink: I was a stranger, and ye took me in: [36] Naked, and ye clothed me: I was sick, and ye visited me: I was in prison, and ye came unto me. [37] Then shall the righteous answer him, saying, Lord, when saw we thee an hungred, and fed thee? or thirsty, and gave thee drink? [38] When saw we thee a stranger, and took thee in? or naked, and clothed thee? [39] Or when saw we thee sick, or in prison, and came unto thee? [40] And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me. [41] Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels: [42] For I was an hungred, and ye gave me no meat: I was thirsty, and ye gave me no drink: [43] I was a stranger, and ye took me not in: naked, and ye clothed me not: sick, and in prison, and ye visited me not. [44] Then shall they also answer him, saying, Lord, when saw we thee an hungred, or athirst, or a stranger, or naked, or sick, or in prison, and did not minister unto thee? [45] Then shall he answer them, saying, Verily I say unto you, Inasmuch as ye did it not to one of the least of these, ye did it not to me. [46] And these shall go away into everlasting punishment: but the righteous into life eternal.
See All... where those who took in the least of his brethren are received and those who rejected them are sent to everlasting judgment. Others teach that works salvation is the meaning of the statement, "he that endureth to the end shall be saved" (Matthew 10:22And ye shall be hated of all men for my name's sake: but he that endureth to the end shall be saved.
See All...). [Though I believe this last teaching refers to the coming of the Saviour at the end of the tribulation and not to eternal life.] There are many other verses in dispute as well.

I studied this in great detail at one point hoping to come up with a complete solution. Many of the questionable verses and passages are not a problem to me now, but I admit that others are. In the end, I fell back on doctrinal foundation. We have a strong doctrinal understanding of how we are saved today. The doctrines of justification, propitiation, regeneration, redemption, and more, carefully expound the spiritual mechanisms by which we come to have eternal life. However, I have yet to find any such understanding in those who teach that the tribulation saints will be saved by faith and works. Everyone I have talked to or read concerning this subject agrees that faith is necessary. However, no one has explained how faith and works operate together on the spiritual plain in doctrinal terms. Some say that they must hold on by works after they have been saved by faith. But this destroys the meaning of faith. Passages in Romans and Galatians contrast faith and works as means of salvation. Faith, however, works by grace (Romans 4:16Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,
See All...); yet, grace and works cannot work together to save a soul (Romans 11:6And if by grace, then is it no more of works: otherwise grace is no more grace. But if it be of works, then it is no more grace: otherwise work is no more work.
See All...).

Yes, works are expected of the saints of the Old Testament. However, I sometimes feel that we forget that they are expected of us as well. We are not saved by works, but it is by works that we demonstrate to others that we are saved. I do not have time to go into the multitude of verses teaching this. Some say that the book of Revelation does not teach salvation by faith in Jesus Christ. It may not be the main doctrinal emphasis of the book , but consider these verses:

Revelation 1:5And from Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood,
See All... And from Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood, 

Revelation 5:9And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation;
See All... And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation;

Revelation 14:12Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.
See All... Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.

Note: those who teach works salvation point out the mention of "the commandments of God." However, this is not a plan of salvation. Rather, it is a description of these people. The "faith of Jesus" points to reconciliation, redemption, justification, and regeneration.

Revelation 21:6And he said unto me, It is done. I am Alpha and Omega, the beginning and the end. I will give unto him that is athirst of the fountain of the water of life freely.
See All... And he said unto me, It is done. I am Alpha and Omega, the beginning and the end. I will give unto him that is athirst of the fountain of the water of life freely.

Revelation 22:17And the Spirit and the bride say, Come. And let him that heareth say, Come. And let him that is athirst come. And whosoever will, let him take the water of life freely.
See All... And the Spirit and the bride say, Come. And let him that heareth say, Come. And let him that is athirst come. And whosoever will, let him take the water of life freely.

The book of Revelation was not written in order to give God's plan of salvation for anyone. The verses quoted above are about as close as you get in the book to a plan of salvation. Yet, none of these verses tell a person how to be saved. For this, we need to go to John or Romans or other similar passages. However, these verses show an understanding of faith in Jesus Christ and redemption through His blood. These concepts simply do not mesh with any kind of works salvation.

I admit that there are doctrinal problems that must be studied to be understood. However, I am often amazed at those who think all the doctrinal problems are solved by calling the salvation of the tribulation a salvation of faith and works. Unknowingly, they create more problems than they solve. Salvation in the tribulation is probably not like that of today in every little detail. For instance, taking the mark of the beast evidently condemns one to hell. However, this does not mean that someone can lose their salvation. Most likely, all true believers will be given the strength by the Lord to reject the mark. This interpretation is borne out in the passages in Revelation about the mark. The time of the tribulation is a remarkable time and many things are different, but salvation is still by faith in Jesus Christ. That fact in and of itself rules out works as a means of salvation.

Salvation in every dispensation is based on three things. One, there must be faith in what God has revealed as the means of salvation. Second, the blood of Jesus Christ must be applied for entry into heaven. Third, the gift of salvation is entirely a work of the grace of God. Nothing is earned. These three principles would also apply to those saved in the tribulation.