Dürer died in 1528, before it was clear that the split of the Reformation had become permanent, but his pupils of the following generation were unable to avoid taking sides. Most leading German artists became Protestants, but this deprived them of painting most religious works, previously the mainstay of artists' revenue. Martin Luther had objected to much Catholic imagery, but not to imagery itself, and Lucas Cranach the Elder, a close friend of Luther, had painted a number of "Lutheran altarpieces", mostly showing the Last Supper, some with portraits of the leading Protestant divines as the Twelve Apostles. This phase of Lutheran art was over before 1550, probably under the more fiercely aniconic influence of Calvinism, and religious works for public display virtually ceased to be produced in Protestant areas. Presumably largely because of this, the development of German art had virtually ceased by about 1550, but in the preceding decades German artists had been very fertile in developing alternative subjects to replace the gap in their order books. Cranach, apart from portraits, developed a format of thin vertical portraits of provocative nudes, given classical or Biblical titles.