Why do you believe the great apostasy and the man of sin is revealed
before the Tribulation?
Daniel 11:36
36 “Then the king shall do according to his own will: he shall exalt and magnify himself above every god, shall speak blasphemies against the God of gods, and shall prosper till the wrath has been accomplished; for what has been determined shall be done.
Daniel 9:25
25 He shall speak pompous words against the Most High,
Shall persecute the saints of the Most High,
And shall intend to change times and law.
Then the saints shall be given into his hand
For a time and times and half a time.
Daniel 13:5-7
5 And he was given a mouth speaking great things and blasphemies, and he was given authority to continue for forty-two months. 6 Then he opened his mouth in blasphemy against God, to blaspheme His name, His tabernacle, and those who dwell in heaven. 7 It was granted to him to make war with the saints and to overcome them. And authority was given him over every tribe, tongue, and nation.
Here he blasphemes God and makes war with the saints for a time, times and half a time, or 42 months. Sounds like the Tribulation to me.
Compare these with the man of sin:
2 Thessalonians 2:3-4
3 Let no one deceive you by any means; for that Day will not come unless the falling away comes first, and the man of sin is revealed, the son of perdition, 4 who opposes and exalts himself above all that is called God or that is worshiped, so that he sits as God in the temple of God, showing himself that he is God.
I believe that the time the beast has authority over the saints to kill them is the last 3.5 years of the seven year Tribulation. I believe he will appear at the beginning of the Tribulation as the conqueror of the first seal, but he will not start out by putting Christians to death. I don't believe the beast of the earth will appear until the middle of the Tribulation and that he will deceive the world and cause them to worship the first beast, putting to death anyone who does not. (He enforces the mark of the beast.)
(Concerning the beast of the earth, or the false prophet):
Revelation 13:12-15
12 And he exercises all the authority of the first beast in his presence, and causes the earth and those who dwell in it to worship the first beast, whose deadly wound was healed. 13 He performs great signs, so that he even makes fire come down from heaven on the earth in the sight of men. 14 And he deceives those who dwell on the earth—by those signs which he was granted to do in the sight of the beast, telling those who dwell on the earth to make an image to the beast who was wounded by the sword and lived. 15 He was granted power to give breath to the image of the beast, that the image of the beast should both speak and cause as many as would not worship the image of the beast to be killed.
A significant part of the Tribulation is people falling away from the faith because of false Christs, persecution, false returns of Christ and false prophets (Matthew 24). The parable of the virgins suggests to me that possibly half of Christians will commit apostasy. Jesus laments concerning the scarcity of the elect at His return, implying that many will forsake Him because they think He's taking too long to avenge them:
Luke 18:7-8
7 And shall God not avenge His own elect who cry out day and night to Him, though He bears long with them? 8 I tell you that He will avenge them speedily. Nevertheless, when the Son of Man comes,
will He really find faith on the earth?”
And many scoffers will stop believing in a literal, bodily return of Christ (2 Peter 3:3-9) because he's taken longer than they thought and because foundationally they rejected the clear teachings about the beginning (God's creation and the worldwide flood) so they don't believe the clear teachings about the end.
Because we know that in Daniel's 70 weeks prophecy, each week is 7 years (this was true of the first 69 weeks which were fulfilled), so we believe the last week is also 7 years and is yet to come--the time of the Jews (where primarily Jews or Israelites are being saved) just before the Lord's return.
Daniel 9:27
27 Then he shall confirm a covenant with many for
one week;
But
in the middle of the week
He shall bring an end to sacrifice and offering.
And on the wing of abominations shall be one who makes desolate,
Even until the consummation, which is determined,
Is poured out on the desolate.”
This divides the 70th week in half - one where he makes a covenant and the second where he brings an end to sacrifice and offering. It could mean there will be a literal temple in Jerusalem, but not necessarily. It could just mean that it becomes illegal to practice Christianity. The abomination of desolation could be desecrating a new temple in Jerusalem or maybe it refers to blasphemy of the Holy Spirit I think will occur if Christians take the mark of the beast in their bodily temple. Half of 7 years is 3.5, which is equal to a time, times, and a half a time or 42 months.