"Dear children, this is the last hour; and you have heard that the antichrist is coming, even now many antichrists have come. This is how we know it is the last hour. They went out from among us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us." - 1 John 2.18, 19.
"Who is the liar? It is the man who denies that Jesus is the Christ. Such a man is the antichrist - he denies the Father and the Son." - 1 John 2.22.
"but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world." 1 John 4.3.
There are many religions today that teach some things that are true, but they all fall short because they do not acknowledge Jesus as the Son of God and the Messiah. These are operating under the spirit of antichrist and when the "man of sin" finally appears, these will acknowledge him without trouble.
Daniel writes that he will make a seven year treaty with Israel that allows them to rebuild their temple in Jerusalem, but in the middle of the treaty time he will desecrate the temple and stop the worship there setting up instead an abomination of desolation - Daniel 9.27. As Paul wrote, he will set himself up as God and demand worship - 2 Thessalonians 2.4.
The spirit of antichrist will be so prevalent at that time that many will be deceived. Many who claim to be Christian will have accepted a false gospel and be unable to distinguish truth. The antichrist will be seen as a great man of peace and by virtue of his miracles he will be heralded as the Messiah. Is peace wrong? No, if it the peace which God brings, but, yes, if it the peace which man brings. Are miracles wrong? No, if they exalt God and lead to Christ, but, yes, if they exalt man.