The best answer here would be pencil.
Artists use pencil to make sketches and create studies. They then use these sketches and studies to create their final larger piece. They use pencil because it's quick, can be corrected, lightweight and easy to use. Before pencils artists would make charcoal drawings but once we had pencils it made sense to use them as they aren't messy like charcoal.
It is true that t<span>he first Christian images are complex and narrative.
You have to understand that this was a time when not many people actually knew how to read - reading was taught only to the rich. This is why many peasants couldn't read the Bible or be religiously educated. For this purpose, there were many Christian images which explained scenes from the Bible, giving a certain narrative to educate the crowds.
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