The Anabaptists saw the Mark of the Beast as infant baptism. This is because the government came to use it as the basis for citizenship, and no man could buy and sell unless he was baptized as an infant.
If one did not profess belief in infant baptism, as we see with Hubmaier, he was subject to being burned at the stake.
“And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads: and that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.”
In Anabaptist theology, perhaps, the first beast would be the devil, and his government of Christendom headed by a pope, and the papacy makes the people worship the Papal Christendom empire, with its Emperor subject to the Pope as final authority.
“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.” –Revelation 13:12
The Beast was seen as that which was killing the Anabaptists.
“And it was given unto him to make war with the saints, and to overcome them: and power was given him over all kindreds, and tongues, and nations.”
For the Anabaptists, “Infant baptism was the ‘mark’ or ‘character of the beast’ in violation of Christ’s fundamental law of church constitution (Acts 8:37, Acts 10:47–Ed)” –from the book English Baptist Reformation: (From 1609 to 1641 A.D.) found at this link–