The Belgic Confession mentions Anabaptists three times: in Articles 18, 34 and 36. The first deals with their denial of the Incarnation; the second with their condemnation of the baptism of the infant children of believers; the third with their rejection of civil government.
A question to be asked is: Is there a common idea behind these three positions? I think there is. The common element must be the rejection of the teaching that God can and does use created stuff to bring about good things.
In Article 18 of the Belgic Confession, we note as a heresy the denial of the incarnation of the Son of God. The Anabaptists denied that Christ assumed human flesh from his mother. They said that Christ took with him from heaven a substance that (only) appeared to be human flesh. They held that Christ passed through Mary like water passes through an eavestrough or like sunlight passes through a window pane. The water takes nothing from the trough; the sunlight takes nothing from the pane. This is the ancient heresy of Docetism.
The Bible teaches, and our confessions repeat, that God did in fact bring something good out of created matter. The Lord Jesus Christ has his humanity from his mother Mary. His flesh was real, true human flesh.
Article 34 notes that Anabaptists condemn the baptism of the little children of believers. They reject the teaching that the infant children of believers ought to be baptized by virtue of the covenant that God has with believers and their children. They do not think that God can bring about something good in the way of the very human line of believing grandparents, parents, children, etc.
The Bible teaches, and the confessions repeat it, that God has a covenant with believers and their children. Baptism, as a sign of the covenant, must be administered to all with whom God has a covenant. This holds for children who are physically born to believers; as well it holds for children adopted into the earthly household of believers.
Article 36 says that the Anabaptists rejected civil government. They did not believe that God could bring about something good by way of human government.
The Bible teaches and the confessions echo that God establishes earthly, human government. He is pleased to govern us, Christians included, by human officers and magistrates.
The Anabaptists rejected that God could do a good work within the realm of nature by way of created, natural, material. They were wrong on all three counts.
Christ has his flesh from the Virgin Mary; the children of believers are comprehended by the covenant of grace together with their parents; God is pleased to govern us by way of human government.