English grammar 'church'..
Same as 'the dog'... 'the cat' .. 'the horse'
Singular standing for plural.
'The horse is a magnificent animal'.. this is non-specific to one horse.. but is representing all horses. Normal English grammar.
Same thing applies to 'the church'.. when it is not specific to one congregation in particular.. it is in this sense of singular standing for plural.
You're ignoring God's word and defending some ridiculous nonsense you read in a booklet. I don't know what to say if you're not interested in listening to His word.