Menno's first knowledge of the concept of "rebaptism", which he said "sounded very strange to me", came in 1531. This came through the means of hearing of the beheading of Sicke Freerks Snijder at Leeuwarden for being "rebaptized". A renewed search of the scriptures left Menno Simons believing that infant baptism is not in the Bible. He discussed the issue with his pastor, searched the Church Fathers, and read the works of Martin Luther and Heinrich Bullinger. While still pondering the issue, he was transferred to Witmarsum. Here he came into direct contact with Anabaptists, preaching and practicing believer's baptism. Later, some of the Münsterite disciples came there as well. While he regarded them as misled and fanatical, he was drawn to their zeal and their view on the Bible, the Church, and discipleship. When his brother Pieter was among a group of Anabaptists killed near Bolsward in 1535, Menno experienced a spiritual and mental crisis. He said he "prayed to God with sighs and tears that He would give to me, a sorrowing sinner, the gift of His grace, create within me a clean heart, and graciously through the merits of the crimson blood of Christ, he would graciously forgive my unclean walk and unprofitable life… Menno Simons rejected the Catholic church and the priesthood on 12 January 1536, casting his lot with the Anabaptists. His date of baptism is unknown, but by October of 1536 his connection with Anabaptism was well-known. In that month Herman and Gerrit Jans were arrested and charged with having lodged Simons. He was probably baptized not long after leaving Witmarsum in early 1536. He was ordained around 1537 by Obbe Philips. Obbe and his brother, Dirk Philips, were among the peaceful disciples of Melchior Hoffman (the more radical having set up the kingdom in Münster). It was Hoffman who introduced the first self-sustaining Anabaptism to the Netherlands, when he taught and practiced believers' baptism in Emden in East Frisia. Menno Simons rejected the violence advocated by the Munster movement, believing it was not Scriptural. His theology was focused on separation from this world, and baptism by repentance symbolized this.
Menno evidently rose quickly to become a man of influence. Before 1540, David Joris, an Anabaptist of the "inspirationist" variety, had been the most influential leader in the Netherlands. By 1544, the term Mennonite or Mennist was used in a letter to refer to the Dutch Anabaptists.
Twenty-five years after his renunciation of Catholicism, Menno died at Wüstenfelde, Holstein, and was buried in his garden. He was married to a woman named Gertrude, and they had at least three children, two daughters and a son.
Menno Simons influence on Anabaptism in the Low Countries was so great that Baptist historian William Estep suggested that their history be divided into three periods: "before Menno, under Menno, and after Menno". He is especially significant in coming to the Anabaptist movement in the north in its most troublesome days, and helping not only to sustain it, but also to establish it as a viable Radical Reformation movement.