Why would Blake's poem "Garden of Love" have been considered revolutionary and controversial in its day?

To answer this question, let us consider what this poem is
about, and more importantly, how it criticises establishes religion. Notice how the poem
describes a visit by the speaker to "The Garden of Love," which now has a "chapel" built in the
midst of it, where the speaker as a child used to play. We can infer therefore that the "The
Garden of Love" was a happy, carefree place of enjoyment and pleasure. However, now, the buidling
of the chapel seems to have converted it into a place devoid of pleasure. Instead, on the chapel
there is a sign saying "Thou shalt not." Turning away, instead of the flowers that the speaker
remembers in this Garden, all he sees are "tomb-stones." The last two lines in particular convey
the criticism of Blake of established religion:

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And priests in black gowns were walking their
rounds

And binding with briars my joys and
desires.

The overwhleming force of
this poem is the way in which religion, symbolised in the "chapel," and its ministers, symbolised
in the "priests in black gowns," choke our "joys and desires" with their religion and creed and
turn joy into ashes. We can understand therefore why such overt criticism of the church would
have been controversial in its day.