Very simple shooter game with a lot of potential to be a high-intensity arcade fiesta! Let's break it down,

The Good:

Whether you were doing it intentionally or not, the lack of a reload mechanic actually cements a lot of what I'm feeling from this game. Notably, a highly arcade approach to the game, like the shoot-em-up games. This is accompanied by the relatively fast movement speed the character has relative to the enemies that we fight.

The fact that bullets can pass through all barriers lends itself to a no-hand-holding kind of feeling, where there is no safe zone, which is interesting to explore. Depending on how you want to go about this game, you can continue the way you're doing and it'd become a bullet-hell esque game (where movement matters more than shooting), or a DOOM esque movement-shooters (where shooting matters equivalently to moving) if you incorpotate barriers to bullets.

So for the core gameplay loop, it's actually quite well cemented, with a good amount of enemy variety that helps flesh things out and give it good meat! Good job!

To improve:

I would love for you to lean onto this high-stakes, arcade feeling. You can do things like, adding a combo meter and a score mechanic, where killing multiple enemies at once gives you more combo and a highscore to beat gives replayability. This will actually lean in well with your premise of controlling a devil going against the (surprisingly biblically accurate) angels, where the devil is extremely violent and power-mad.

Also be mindful of the gun level up systems, make sure to balance them well as I find the AK at level 4 and above is extremely OP.

If you're going down this high-stakes arcade feeling, I'd recommend going for more close-quarter weapons like shotguns, and have the player go up close and personal to encourage intensive encounters. 

And by doing so, the player may make mistakes, so you can help the player recover their mistakes by dropping health kits every so often as rewards for performing well.

Solid game! ULTRA PREMISE!