Hes promoting Christianity as the truth here anons. Reading comprehension is critical. Now whether you believe Christianity is the truth or not is up to you, but trying to twist this around and saying he's anti christian or promoting satanism is only showing your ass.
He's saying the enlightment, which was a secular anti christian movement in general, turned us away from the truth (meaning Christianity).
"The afterglow of Christianity is near gone now" meaning the secular world has nearly completely taken over the world, and is casting a shadow where Christianity once glowed bright.
"A slygian silences lurks in wait" As in a dark, gloomy future is a head of us because we turned away from Christ.