Morning samuel23,
Why would they have to be saints prior to the tribulation? Why couldn't they becomes saints during the tribulation? You are partially correct in that, there most likely be those who claimed to be in Christ, that will have gone back into living according to the sinful nature that will have known the word of God regarding end-time events. Once the church is removed, they would realize what had happened and know what they are going to have to go through in order to be saved. During that time there is also going to be 144,000 servants of the Lord and the two witnesses who will take part in evangelizing the world during that time. There is also going to be an angel proclaiming the everlasting gospel to all of the people of the earth.
People are going to realize that they have entered into the time of God's wrath and will be willing rather to die, than to save their own lives, keeping the testimony of Jesus and the word of God, even unto death.
However, the beast doesn't kill all of them. For if they did, then there would be no Gentile believers to go into the millennial kingdom. Yet, according to the following scripture, we know that isn't truth:
"He will judge between the nations and will settle disputes for many peoples. They will beat their swords into plowshares and their spears into pruning hooks. Nation will not take up sword against nation, nor will they train for war anymore."
The above takes place after Jesus returns to the earth to end the age and at the on-set of the millennial kingdom. Therefore, not all of the great tribulation saints are killed.