I need to disagree with most commentators on this poem. Certainly, this poem is about how the church opposes the "natural" state of "love" (at least, sexual love); yet I believe that Blake, like most romantic poets, was almost totally wrong.
First, they put up the bogeyman called "the church" and attack it, yet the church is not original here. Rather, it's the Bible that clearly supports some forms of sexual love and says "thou shalt not" about others.
But the more basic problem is that they assume, without argument, that the current natural state of sexual love is good and that the church (following the Bible) is wrong in stating "thou shalt not." This is far from obvious to me. There's no rational way of proving it. And, regarding our actual experience with life, a simple look at what is now the "natural" state of sexual love indicates this is not so: all the murders and infidelities and treacheries that have been committed in the name of "natural" sexual love, the hundreds of extant venereal diseases.
Blake is right on one point: in the "garden" (assuming he was referring to Eden), natural love was good; but we are clearly not in that garden any more. It's not "the church" that bars us from re-entering: it's your sin and mine. And that's not a simple problem that is easily solved by writing emotional panegyrics on the goodness of unbridled sexual love: it's a problem that's only solved by the ultimate sacrifice, the sacrifice made by Christ on the cross.
In summary, by all means read and analyze the poetry of Blake: but if you want to get to the truth of the matter, I urge you to throw off the easy emotional answers chosen by Blake and look for deeper rational answers. Perhaps you'll find the church (and neither Blake's caricatures of it, nor your own emotions) was really right in the first place.