When Mallais painted this image of Ophelia dead it was considered bad taste to paint dead people. Death was considered the opposite of what art is supposed to be, which is a celebration of beauty. Mallais painted this and it was so beautiful that it changed what could be painted and how it could be presented. For him to present death as something beautiful and peaceful was revolutionary. At the time the church ran everything. Even art was heavily controlled. For Mallais to take back freedom of expression from the church caused others to do it in fields other than painting.
This image is even more controversial. To depict God as a human was shyt they'd burn you at the stake for. This a painting by the same painter depicting a young Jesus. At the time you couldn't depict God touching the floor, without a halo or light, or doing anything human. Also the expressions on the people's faces. Mallais gave people emotion. He also painted with great attention to detail so the entire picture matters, it's not just the subject, which is God. Mallais was saying we are all important and if God has the image of man than he should be depicted as a human.
Paintings were like movies. They'd tour the world and people would talk and write about them. So for Mallais to make a painting challenging the status quo when that wasn't okay encouraged people to do it too.