Lyric discussion by just1comment 

Cover art for Prayer in C lyrics by Lilly Wood & The Prick

I agree with ladypetrichor and rod that it is fundamentally addressed to God.

It could also be interpreted as being addressed to everyone individually who did nothing to stop climate change.

you never said a word never sent a letter (activism or change in behaviour) Don't think I could forgive you

The second verse could be heard as "our word is slowly dying" referring to humanity or governments climate change commitments. Don't think I could believe you

The third verse moves forward in time to when we're older.

And see the children are starving And their houses were destroyed Don't think they could forgive you

Ultimately you won't forgive yourself for doing nothing.