Some people confuse mysticism with mysterious. They’re not the same. Many things are mysterious. But mysticism is an “ism”—based on the idea that we can’t connect with God via our normal senses. Which means, normal Bible reading and prayer are inferior spiritual practices, and so we need to discover spiritual secrets to “go beyond” and “break through” to God. Gary Gilley’s older work on mysticism is still relevant. Go to his website “Think on These Things” for a 4-part in-depth study; part 1 starts here.