It seems that various famous reformers venerated Mary but in my experience there is a general disregard of Mary in modern Protestant denominations. When did this sentiment historically wane?
I'm excluding the Lutheran church, which still uses the title "Mother of God" here.
Protesters Why were the Early Protestants still venerating Mary to an extreme? The answer lies in the very word, Protestant. These men were in the main all ex-Catholic priests. They had been steeped in the Catholic doctrines and teachings, and Customs, and Traditions of Catholicism. But they began to disagree with what they had been raised in, and protested vehemently on what they considered so important.
However they did not dump everything 100% because they were used to all the trappings of Rome, but rather focused on what they considered the essentials of the Christian faith. The history of Protestantism is one of a gradual coming out.
Later in history, the other minor doctrines came under fire by succeeding Protestants who, by this time, never had ties to Rome. They never had been Catholic. These biblical scholars were distant from the trappings of traditional Catholicism. So it was easier for them to downplay the Veneration of Mary without any qualms.
To them Mariology became to be Mariolatry, a type of idolatry. They recognized Mary's own confession of a need for a Savior, in her Magnificat (Luke 1:46-47) so she could not have had an Immaculate Conception. (See also Romans 3:23)
It was in proportion as to how far away from the Roman Church (as well as how far from the initial Protestation era) that modern Protestants distance themselves, that Mary becomes insignificance, especially in her alleged role in a person's salvation. (Acts 4:12 Neither is there salvation in any other, for there is none other Name under heaven given among men, whereby we must be saved.)
Most modern Protestants have never had any contact with Roman Catholicism, so the doctrine of Mary is quite foreign to them, and is considered non-essential to the Christian life. Their focus is solely on the awesome Lord and Savior, Jesus Christ!
The Early Protestant Reformers largely venerated Mary, when did this sentiment change and why?
It was probably around the 17th - 19th centuries that Protestant theology began to change in this regard, due to a change in the way their theological viewpoints on this issue began to be alter as being unfit and possibly looked on as being idolatry. Even this is not clearly defined by all Protestants as their viewpoints are not united in any theological manner or unified set of beliefs.
The first-generation Reformers maintained a view of Mary that most contemporary Protestants would blush at. Martin Luther continued to believe in the perpetual virginity of Mary and her immaculate conception. Calvin was willing to accept her perpetual virginity as at least possible and criticized other Protestants for rejecting the Catholic doctrine out of hand. One may speculate as to whether these Reformers’ retention of certain Catholic conceptions of Mary was motivated more by residue from their own upbringing or their reading of Scripture.
Protestant views on Mary include the theological positions of major Protestant representatives such as Martin Luther and John Calvin as well as some modern representatives. While it is difficult to generalize about the place of Mary, mother of Jesus in Protestantism given the great diversity of Protestant beliefs, some summary statements are attempted.
Karl Barth
Karl Barth (1886–1968), a Reformed Protestant, was a leading 20th century theologian. Aware of the common dogmatic tradition of the early Church, Barth fully accepted the dogma of Mary as the Mother of God. In his view, through Mary, Jesus belongs to the human race; through Jesus, Mary is Mother of God. Barth also agreed with the Dogma of the Virgin Birth. It meant to him that Jesus as a human does not have a father and that as the Son of God he has no mother. The Holy Spirit, through whom Mary conceived, is not just any spirit, but it is God himself whose act must be understood spiritually and not physically. Mary is "full of grace" according to Barth, but this grace is not earned but totally given to her. Regarding Mary's virginity after birth, Barth argued that the Church adopted this position not because of Mary but in defence of its Christology. Barth considered the Roman Catholic veneration of Mary a terrible mistake and idolatrous heresy.
Mariolatry
From the 17th to the 19th centuries various groups of neo-Protestants began to use the term Mariolatry to refer to the Catholic, Lutheran, Anglo-Catholic, Oriental and Eastern Orthodox practices of Marian veneration and devotion. According to their researches, the attention paid to Mary is extreme, and may not only distract from the worship of God, but actually be idolatry.
This trend has taken various directions over time, in that while some Protestants have at times softened their attitude towards it, others have strengthened their opposition in the 21st century. For instance, during the May 2006 celebrations at Our Lady of Walsingham in England, as Anglicans and Catholics held a Marian procession, nonconformist Protestant hecklers held banners that condemned Masses, idolatry, and "Mariolatry". - Protestant views on Mary
The following may be of interest to some:
You say “they still use the title Mother of God”, but in fact most protestant denominations agree on that. Maybe they don’t use the title, but the dogma of Maria Θεοτόκος is not about Mary in the first place.
Maria Theotokos, the fact that Mary is the mother of God, says something about Jesus in the first place: that when He incarnated, took the flesh through Mary, He was 100% God and 100% man. Not 50%+50%, not “man but for the time being not God”, not any other variant.
Most protestants, all catholics and orthodox churches teach this. So even if protestants don’t use to call her Mother of God anymore, they still believe or teach the same facts.
