Astell seems to be using sublime in the sense "Of a person, personal attribute, action, etc.: morally, intellectually, or spiritually superior; of great nobility or grandeur. Hence: perfect, consummate; supreme" (usage starting around 1600)
Not to be confused with the later concept of the sublime (starting around 1750, which I'm familiar with in reference to Romantic literature) as "Of a feature of nature or art: that fills the mind with a sense of overwhelming grandeur or irresistible power; that inspires awe, great reverence, or other high emotion, by reason of its beauty, vastness, or grandeur."
http://www.oed.com.ezp.slu.edu/view/Entry/192766?rskey=35FZPc&result=1&isAdvanced=false#eid