Add to it that painters of that period often just used the religious theme as an excuse to paint. Up until the modern era The Church frowned on anything without a religious theme. During the renaissance they loosened up to allow for classical themes as well but you were safer keeping it to religion.
I would say prior to the Renaissance the church was the institution with the financial means to commission and pay for art for art's sake. And that therefore they (generally) controlled the subject matter being produced. They were also best placed to safeguard art throughout the centuries, prevent it from being inherited by the disinterested to afterwards be discarded, neglected and deteriorate.
By the Renaissance though, individuals (traders, bankers) were becoming so rich and affluent that they themselves could start commissioning art suited to their own personal tastes. This often took the form of portraits but then spilled over into other subject matter, like a still life, special occasions from everyday life (think of a Breughel village/marriage feast), perhaps something mythological that allowed the artist to show a serious amount of naked flesh or a plethora of nude nymphs or goddesses from Antiquity. Think of the last as antique soft porn … ha ha.
But in any case, those secular groups becoming wealthier were then able to call the artistic shots more and more, because they controlled wealth.
And one of my personal notions is that so so much has gone lost over the centuries, especially art in what might be called the lower end of the price range.
We also have to take into account what pieces that museum curators and directors allow us to see in exhibitions or in published art books. They go for the big attractions and money-makers so it always tends to be the same-old, same-old throughout the decades/centuries. Every now and then a book on art will be published containing rarely seen pieces, but that doesn't happen enough. So our notions of what art was in centuries past is based on the images we see again and again. Perhaps a bit outside of the time frame but on Pharaohonic art, we seldom see anything that is not monumental or impressive. But there are things like dawdles or sketches made of irreverent subjects that have been found, like copulating couples, a sex party etc. These are often thought inappropriate for mass consumption.