Why were religious paintings created?
When I was studying for the Level 4 Art Certification exam, I read several introductory books on Western art. Among them, the explanation of why religious paintings were created was particularly convincing.
When you hear the term "religious painting," what kind of artwork do you imagine? Actually, this painting is also a religious painting.

Why were such religious paintings created? The answer was written in this book.

It is written on pages 98-99 of this book as follows.
The first role
...The first role is to help people who cannot read the Bible learn about faith by showing them pictures based on the Bible.
In other words, religious paintings were created intentionally to help people learn about religion. Here, the important point is, 'Why express it through paintings?'.
This has a lot to do with the language of the Bible at that time. The Bible at the time was written in Latin. And it was something that only the educated, privileged class could read and write. In other words, it was a distant language for the common people. If it could not be conveyed through language, they would express it using paintings that appeal visually. It was against this background that religious paintings came to be created.
The second role
The second role is to give the stories of saints and others a vivid sense of reality by showing them to people who do not feel faith even when they hear the stories, thereby evoking faith.
In order to prevent a lack of faith, such as wondering if Christ really existed, using visual religious paintings to give the stories a sense of reality was also one of the purposes for which religious paintings were created.
The third role
And the third role was to make stories related to Christianity, which are difficult to keep in memory even when heard, visually impressive by showing them as paintings, so that they would be remembered for a long time.
Finally, there is memory. Bible stories that you cannot remember after hearing them once, or that you only vaguely remember, can be viewed repeatedly if preserved in religious paintings, so they remain in people's memories.
In other words, religious paintings were an essential means for instilling faith in people, especially in the Middle Ages when Christianity was dominant. That is why they placed very easy-to-understand markers.
For example, just as a painting with a red hat and a lion can be identified as Saint Jerome, conventions were established for religious paintings. By doing this, even common people, for instance, could look at a painting and understand, "This is a painting of the Trinity."
How is it different from an allegorical painting?
As we have seen so far, religious paintings are, so to speak, "paintings for everyone." Rather than confusing the viewer with obscure paintings, the important point of religious paintings was that they were straightforward, allowing one to immediately understand what Christian story was being conveyed.
On the other hand, there are also allegorical paintings (or allegories). These are also called allegories. For example, this painting is an allegorical painting called "An Allegory of Love."

This painting is filled with many meanings. Each person symbolizes something, and if you do not know what each thing signifies, you will not be able to understand what the message of this painting is. Rei Nakamura describes allegorical paintings as follows:
...(Allegorical paintings) required a certain amount of experience to be able to perform a "correct interpretation" on one's own. (P172)
...They are paintings with an exclusive nature, where it does not matter if they are not understood by those who cannot comprehend them. (P176)
From this, we can consider that religious paintings and allegorical paintings have diametrically opposed purposes. Doesn't just knowing the reasons why paintings were created in this way make art feel a little more accessible?
