Sources please. If it is “many”, finding many sources should be easy.Because - and this is an unpleasant answer - many Protestants believe Constantine (not Jesus) founded the Catholic church, and as such, they don’t want to learn what’s to them “pagan” history. When Constantine’s no longer taboo and is openly talked about, I think that’s when you’ll find more Protestants wanting to learn Catholic Church history.
Here’s one. The LCMS Calendar of Commemorations.
JonMay 21
Constantine, Emperor, and Helena, his mother
Constantine I served as Roman Emperor from A.D. 306 to 337. During his reign the persecution of Christians was forbidden by the Edict of Milan in 312, and ultimately the faith gained full imperial support. Constantine took an active interest in the life and teachings of the church and called the Council of Nicaea in 325 at which orthodox Christianity was defined and defended. His mother, Helena (ca. 255-329), strongly influenced Constantine. Her great interest in locating the holy sites of the Christian faith led her to become one of the first Christian pilgrims to the Holy Land. Her research led to
the identification of Biblical locations in Jerusalem, Bethlehem, and beyond, which are still maintained as places of worship today.