How do we know that Satan is not the Antichrist? Satan is the dragon and the beast is his Antichrist. It is easy to see in the following verses that the beast (Antichrist) and his false prophet are captured and thrown in the lake of fire first while Satan is bound for a thousand years and does not get put away permanently until after the time of testing and war that will happen after the Millennium.
First, we see in the following verse the fate of the beast/Antichrist & the false prophet.
Revelation 19:20 Then the beast was captured, and with him the false prophet who worked signs in his presence, by which he deceived those who received the mark of the beast and those who worshiped his image. These two were cast alive into the lake of fire burning with brimstone. (at the end of the 7 years)
After the millennium, we then see Satan, the devil is cast into the fire with them, showing that they are three separate entities.
Revelation 20:10 The devil, who deceived them, was cast into the lake of fire and brimstone where the beast and the false prophet are. And they will be tormented day and night forever and ever.
Interesting and intertwining note:
Satan will have two great battles with God. Most people realize there will be a battle at Armageddon but the final battle seems to be seldom spoken of.
1. The first great battle is at the end of the 7 years (Armageddon). See: Revelation 19:20, Revelation 16:14-16. The beast and false prophet are cast in to the burning lake of fire at this time.
2. The second great battle is after the 1000 year reign of Christ. See: Revelation 20:7-10. Satan having been bound for a 1000 years, is set loose for a short season. He rallies war against Jesus and the saints, but is captured and cast in to the lake of fire where the beast (Antichrist) and false prophet will already be.
But you, brethren, are not in darkness, so that this Day should overtake you as a thief. You are all sons of light and sons of the day. We are not of the night nor of darkness. Therefore let us not sleep, as others do, but let us watch and be sober. 1 Thessalonians 5:4-6
While we do believe in a mid tribulation rapture, we are not saying we know the day or the hour. The Lord makes it very clear that no one but the Father knows that. While we do believe it is at or near the middle of the last 7 years of this age, the Lord says that He has to cut the time short for the elect's sake in Matthew 24:22 and who knows the amount of time that will be?