One of the prayers for Christmas morning asked God to “illumine” us or some people or the whole world (I can’t check because I didn’t bring the service sheet home). Illumine and illuminate are both valid English words. Both are from Latin illuminare (verb) and lumen (noun). According to Dictionary.com, illumine is earlier (1300-50), but it is defined only as “to illuminate”. Illuminate dates from 1400-50, and –ate is certainly a more common verb ending.
Illumination covers actual and metaphorical light. Sometimes the Bible talks about actual light and sometimes about metaphorical light, and sometimes it is hard to know which is meant. This morning’s Gospel reading was from John 1, which includes “In him was life, and that life was the light of men. The light shines in the darkness, but the darkness has not understood [or overcome] it” (vv4-5 NIV, see also vv8-9).
Even though illumine is the older form, Google Ngrams shows that it was very rarely used until about 1700. From about 1750 to 1900, it hovered around a quarter of the frequency of illuminate, after which illuminate has grown and illumine has declined in use.
To me, illumine sounds more poetic, and more metaphoric (no doubt the growth of actual illumination after 1900 largely accounts for the growth of illuminate). While it is possible to ask God to (metaphorically) illuminate us, it would be very unlikely for a movie director or director of photography to ask the gaffer (chief lighting technician) to (actually) illumine the set in a certain way.
Unlike preventive and preventative, where I would unhesitatingly recommend the shorter alternative, here I would recommend the longer version, illuminate.