radix

Radix, radicis, f., third declension. The literal sense is a plant root, and from there it takes one short step to the foot or base of anything solid: sub radicibus montis, at the foot of the mountain, is standard in the historians for where an army camps.

The metaphorical step is just as short - the origin or source of a thing - and that is the sense the ecclesiastical corpus wants for radix malorum. The word travels well: Spanish raiz, French racine and Italian radice, and English radish, radical, eradicate and, through a Norse detour, root itself.