"Churches are religious buildings."
Translation:Les églises sont des bâtiments religieux.
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".