Menno Simons never intended to found a denomination, but the movement his preaching organized needed a name, and by the 1540s Dutch documents were already calling his followers Mennists, a label that hardened into Mennonite and has outlasted every debate about whether he earned it. His own writing is where the tradition's convictions were first set down at length, above all in Foundation of Christian Doctrine, published in 1539 and 1540, and in the shorter Why I Do Not Cease Teaching and Writing of the same year.
Three commitments run through that body of work. Baptism belongs to those old enough to confess their own faith, not to infants received into a Christian society by default. A visible church has to be able to correct its own members, applying the ban, a form of church discipline withdrawing fellowship from someone who will not repent of open sin, so that the congregation's discipleship means something. And the church has to stand apart from the state's violence, refusing the sword, the oath and the compromises that came with established religion.
Menno also held an unusual view of Christ's incarnation, teaching that Christ took on a heavenly rather than an ordinary human flesh from Mary, a position that put him at odds even with some fellow Anabaptists and that later Mennonite theology has mostly set aside. What survived him was not that argument but the discipline, the believer's baptism, and the name, still carried today by Mennonite communities descended from the congregations he gathered four centuries ago.