The sun is the most massive thing in nearby space. we're flying around it fast enough that it's gravity- a result of all that mass- isn't able to drag us in. this is called an 'orbit'. The Earth, is orbiting the Sun and has been for a very, very long time. we call a single orbit a 'year'.
Now, while the Earth is orbiting the Sun, and we're all doing stupid human tricks, the Earth is also spinning like a top. This rotation is what causes our day and night cycles. As we stand on a single place on the Earth, it looks like the sun is travelling across our sky because we're spinning around.
It's like being on a Carousel going round and round. As you go around, you can see people in line waiting to get on, and then you go past, and you can't see them any more. and then you come back around and you see them again.
Seriously. The sun sustains all life. And it may very well be to thank for the original creation of life. Basic amino acids form spontaneously in dirty water that’s exposed to sunlight. When the first single celled organism spawned it was surrounded by food. No one ever really asks what the first lifeform ate, even though that’s a very good question since most life depends on other life for food. This is the answer to that question.
The Sun is also absolutely terrifying. From a million miles away its rays give warmth but it is a gargantuan explosion that’s only held together by how huge it is. The sun is so massive and seething that sometimes it takes photons produced by solar fusion hundreds of thousands of years to wiggle their way out to the surface. The sun is too terrible to stand before. Truly awe inspiring in its majesty and absolutely beyond our ability to even fucking touch.
You know. All those things that god is supposed to be.
This is a point brought up in Scorcese's Silence (maybe also in the novel written by Endo, which the film was written from) :
Ferreira: Francis Xavier came here to teach the Japanese about the son of God. But first he had to ask how to refer to God. "Dainichi", he was told. And shall I show you their Dainichi?
[He points to the sun in the sky]
Ferreira: Behold... there is the SUN of God. God's only begotten sun. In the scriptures Jesus rose on the third day. In Japan... the sun of God rises daily.
context of the movie
Portuguese Christians receive the news that a high priest in Japan - sent to convert the Japanese - has renounced his faith, they send three priests to find where he is