Many of the books discussed on PEL advance their thesis methodically. Not so with Schleiermacher’s On Religion. (Schleiermacher’s approach changed after he became a university professor, whereupon he became more systematic and less interesting.) Schleiermacher’s lack of structured argument may have resulted from his theological, as opposed to philosophical, training. But it’s also a function of his audience. On Religion was not primarily a philosophical tract, but a religious work addressed to a literary circle of cultured atheists. Think McSweeney’s with more money.
Perhaps the young Schleiermacher and his fellow German Romantics were rebelling against the mode of philosophical argument itself. What unified all the Romantics (including Rousseau) was their appeal to sensibility over sense. To better understand Schleiermacher’s influences, listeners might want acquaint themselves not only with Kant’s Critique of Practical Reason, but also the proto-Romantics of the German Sturm und Drang movement. Unlike Kant’s Critiques, I don’t know how well On Religion can be divorced from its cultural context. That’s not to say his arguments are obsolete; Schleiermacher’s emphasis on religious mysticism over religious doctrine is relevant to anyone seeking religion but not a church.
Kenneth Clark’s TV series Civilisation had two good episodes covering the Romantic Era’s prelude and peak, both on YouTube. For a more traditional historical survey, catch the Revolution & Romantics episode (no. 43) from Eugen Weber’s TV program, The Western Tradition. And, as always, Bertrand Russell had something to say on the subject.