"The highest and chief abomination of the pope."
The phrase, describing infant baptism, is from the Schleitheim Articles, the first official confession of faith promulgated in 1527 by a group of Swiss Brethren who taught that baptism was for believing adults only and that infant baptism was invalid. Since all of these dissenters had been baptized as infants—the Reformation was only a decade old at this point—they found it necessary to rebaptize one another and so became known as the Anabaptists (rebaptizers).
Today’s Evangelicals and Fundamentalists concur that baptism should be restricted to adults, but, curiously, they are descended organizationally not from these early Swiss Protestants but from the Anglican church, which was begun by Henry VIII in the decade following the Schleitheim Articles. The Anglican church, like the Lutheran and the early Reformed (Calvinistic) churches, continued baptizing infants. Some of these churches dropped the practice early on, as their increasingly anti-Roman theology developed; others continue the practice still.
What occurred in the early years of the Reformation is well explained by (perhaps surprisingly) The Encyclopedia of Protestantism:
At the time of the Reformation, the sacraments, those ceremonial acts that dramatized biblical events as they served as a sign of God’s presence, became a key issue between Roman Catholics and Protestants and among Protestants themselves. Protestant dropped five of the seven Roman Catholic sacraments and moved to bring the remaining two, the Lord’s Supper (eucharist) and baptism, into line with their new theological perspective. During the sixteenth century, the Lord’s Supper received the bulk of the attention, as the major issue dividing Lutherans, Reformed, and Anglicans. Each continued the Roman Catholic practice of baptizing infants.
For many Protestants that changed in subsequent years. Infant baptism was discarded, much as five of the seven sacraments had been discarded, yet it was not until the 19th century that "believer’s baptism" ceased to be a minority position among Protestants. The credit goes to the Baptist churches, which were growing rapidly just as some of the older Protestant churches slowed in their growth or even began the long, slow decline that in recent decades has ceased to be slow.
An ancillary issue arose: How should baptism be performed? Catholics used chiefly affusion, pouring water over the head while recognizing immersion and even sprinkling. Baptists and others claimed that only immersion was permissible, arguing, by an appeal to the Greek text, that biblical references to baptism command immersion. To such Protestants, baptism by affusion was no baptism at all.
What little biblical evidence there is about the mode of baptism actually points elsewhere than toward immersion. On the first Pentecost, 3,000 were baptized in Jerusalem, which had neither a running stream or a large pool of water. A few moments with a calculator will demonstrate the infeasibility of baptizing so many by immersion in one day, if one posits, as archaeologists do, that those first recruits to the Church could not have been immersed in anything much larger than a watering trough for horses. It is likely that the method used that day was affusion.
This accords with all of the earliest artistic depictions of the baptism of Christ at the Jordan River: Jesus stands ankle deep in the shallow water, while John the Baptist, using a shell, pours water over his head. Scholars have discovered no ancient writings that report claims of misrepresentation, either with respect to our Lord’s baptism or to anyone else’s baptism by pouring of the earliest Christians. Had immersion been the only acceptable practice for the nascent Church, no doubt there would be records condemning "innovators" who took up affusion. But there are no such records because there was no such condemnation—until the Anabaptists came along and reinvented Christian sacramental history.