The Scarlet Letter
Said caveat was once the prattle and banter of mere boys, with their hands on a wand and The Book of the Law gird like swaddling to hips that would one day wear well a magical sword, The Book of the Law long committed to mindful memory. Once the admonition served them well and protected them from they knew not what, but, oh, was it wise then.
However, these men are now true priests. The timbre of their voices sounds in the manly range—hell, far more beastly than those swollen-headed boys ever dreamed possible. Where once their invocations summoned mommy and the local police,* now their rituals effect the desired result, whereupon the gods themselves arrive to take possession and reside.
Never to invoke anything bigger than one's head is to relegate oneself to the lower echelon of spirit forms. Not that spirits small in stature don't, in their brightest moments, expand their own consciousness and, therefore, that of the cosmic whole, when, as mere minions, they serve higher forces. Yes, even little spirits evolve, should it be their will. But Wit is not their will and you have called them down, you get what you deserve. Little spirit forms. Little psycho kinetic subordinates who, when not obliged to exhibit good behavior, are sometimes best tempered in a hot brass box, in which case, the triangle was perhaps more apropo than your head—for better control, mind you.
Never to invoke anything bigger than one's head is, at best, not unlike Siamese twinship of consciousness, with one experience being shared by you and a familiar spirit—good old Great Aunt Harriet or possibly Frater Fred. Worse could be anything from Goetic demons to focused, non-terminal repeating phantasms or Class V free-roaming vapors. Unfortunately, the bugger of it is that in either case, spirits like animation and dig being in a body, so they don't banish as easily as they are invoked, unless they truly love you (which you definitely shouldn't count on in a worst-case scenario).
Anything of a morally superior and divine nature is absogoddamlutely bigger than one's head. Any magician worth his/her weight in cinnabar transformed to gold, any magician in direct communication with her/his HGA probably has the technology and will to invoke divinity.
Definitely. Definitely invoke up.
The truest maxim is, as always, Do what thou wilt, and every invocation will be blessed. But in the course of invoking bonafide deities... remember. Shouldst thou stray from thy Holy Will, prepare thyself for the sheer weight of what it is thou hast brought down on thy cranium. The gods will provide tuition to those who misinterpret the Law. And they only leave when they're ready.
* See The Scales, No. 1, "Magical Mistakes," by Frater Insanvs, © copyright Serpent In Balance Camp, O.T.O., 1996.