[1] Early Christians were wary of publicly exposing the cross or crucifix for fear of subjecting it to the insults of pagans, or scandalizing the weak.
By the start of the 13th century, treatises by Pope Innocent III expected there to be a cross between two candles on the altar during the Mass.
Roman Catholic liturgical norms require a crucifix (with the corpus or body of Jesus) near or on the altar whenever Mass is celebrated.
Lutheran churches retained both altar and crucifix (with the body of Jesus displayed) after the Reformation, and is the norm in Europe.
The influence of the Reformed tradition, which comprises the Protestant majority in North America, many Lutheran churches there prefer a plain altar cross rather than a crucifix.