Matthew 24:21-28 contain Bible verses that happen during the great tribulation according to Matt. 24:21.  

Matthew 24:23-26 says, "Then if any man shall say unto you, Lo, here is Christ, or there; believe it not. For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect. Behold, I have told you before. Wherefore if they shall say unto you, Behold, he is in the desert; go not forth: behold, he is in the secret chambers; believe it not."

Why would someone during the great tribulation period say, "Lo here is Christ or there" when Christ tells us to believe them not. Christ's believers know that He won't be on Earth during the Great Tribulation.  Would it be because the person saying, "Lo here is Christ or there" is deluded and deceived into believing that the beast (Antichrist) is Christ?  Is that the lie that they are deceived into believing by the miracles, & great wonders that the Antichrist & False Prophet perform so that the people who worship him and his image believe a lie that the Antichrist is Christ (the Messiah) and they believe he is performing miracles by the power of the Holy Spirit when in fact the Antichrist also known as the son of perdition, who may call himself Christ, is performing lying wonders and miracles with the power of the dragon who is called Satan.  Is the beast called Christ or does he have Christ as part of his name?

When Jesus cast out devils (unclean spirits) and the Pharisees said he was casting out devils with the power of Beelzebub in Matt. 12:24 & 28, Jesus said that it was blasphemy of the Holy Spirit which is unpardonable because they said Jesus was casting out devils with the power of the Holy Spirit.  Would it also be blasphemy of the Holy Spirit, an unpardonable, unforgiveable sin, for the beast's worshippers to say that the Antichrist, who they think is Christ, and the False Prophet are using the power of the Holy Spirit to perform miracles & great wonders when they are really using the power of Satan to perform miracles & lying wonders?? If this is blasphemy of the Holy Spirit, then this would be an unpardonable sin if the beast's worshippers & those who took the mark of the beast, even wanted to repent.  However, according to Rev. 16:9 & 11 those taking the mark of the beast and worshipping his image do not repent of their deeds.

Below are Bible verses about the power of Satan, (the Dragon) the man of sin (the son of perdition, Antichrist, or the 1st beast) the second beast or False Prophet, and how miracles & lying wonders are used to deceive the people on Earth who worship the image of the beast.

2 Thess. 2:3-13 tells us about the man of sin, the son of perdition & about strong delusion that causes them that perish or the unrighteous to believe a lie. 2 Thess. 2:9-13 says,

"Even him, whose coming is after the working of Satan with all power and signs and lying wonders. And with all deceivableness of unrighteousness in them that  perish; because they received not the love of the truth, that they might be saved.  And for this cause God shall send them strong delusion, that they should believe a lie: That they all might be damned who believed not the truth, but had pleasure in unrighteousness."

Rev. 13: 2-4, "And the beast which I saw was like unto a leopard, and his feet were as the feet of a bear, and his mouth as the mouth of a lion: and the dragon gave him his power, and his seat, and great authority. And I saw one of his heads as it were wounded to death; and his deadly wound was healed: and all the world wondered after the beast. And they worshipped the dragon which gave power unto the beast: and they worshipped the beast, saying, Who is like unto the beast? who is able to make war with him?"  

In regards to the False Prophet, Rev. 13:12-15 says, "And he exerciseth all the power of the first beast before him, and causeth the earth and them which dwell therein to worship the first beast, whose deadly wound was healed. And he doeth great wonders, so that he maketh fire come down from heaven on the earth in the sight of men,  And deceiveth them that dwell on the earth by the means of those miracles which he had power to do in the sight of the beast; saying to them that dwell on the earth, that they should make an image to the beast, which had the wound by a sword, and did live. And he had power to give life unto the image of the beast, that the image of the beast should both speak, and cause that as many as would not worship the image of the beast should be killed."