"Churches are religious buildings."
Translation:Les églises sont des bâtiments religieux.
Des églises sont des bâtiments religieux. (Why this is incorrect here)
Because when we say Churches are ..., it means all the churches or churches in general. This is translated by Les églises in French.
If you said Des églises, it would mean Some churches
As noted below, it is "les" églises because it is a general statement (in English we don't require definite articles for plural nouns, but we do for singular nouns "Snakes are dangerous animals/The snake is a dangerous animal").
To use "les", you would already have to be talking about specific buildings, but this sentence is completely out of context.
"Des" here is NOT "de + les", but the plural of "un".