From Haydock’s Commentary:
Ver. 12. Flesh and blood, which may either signify temptations of the flesh, or raised by mortal men. — Principalities and powers; i.e. devils, or apostate angels, who before their fall were in such ranks of spirits, and who are permitted to rule over the wicked in this world of darkness. (Witham) — By which we are to understand the fallen angels. For as by nature, and from their creation, they were the governors of this corporeal world, and were deprived of this their power on account of their pride, they received it (though limited by certain restrictions) in order to tempt man. (Estius) — Rulers of the world of this darkness. By these are meant the devils who exercise their power and authority in our inferior and dark atmosphere, by raising winds, storms, tempests, &c. By darkness may be understood the wicked, in whom Satan reigns as in a citadel. (Menochius) — Our inferior world is called dark and misty in comparison of the world above, which is always bright, serene, and clear. Our atmosphere is called the cloudy and dark heavenly. Cicero, in his Tuscul Quæst. Prudentius likewise, in Hamartigenia, writes thus:
Non mentem sua membra premunt, nec terrea virtus
Oppugnat sensus liquidos, bellove lacessit;
Sed cum spiritibus tenebrosis nocte dieque
Congredimur quorum dominatibus humidus iste
Et pigris densus nebulis obtemperat aer.
Scilicet hoc medium cœlum inter et infima terræ
Quod patet ac vacuo nubes suspendit hiatu,
Frena potestatum variarum sustinet, ac suo
Principe Belial rectoribus horret iniquis.
His colluctamur prædoribus, at sacra nobis
Oris Apostoli testis sententia prodit. ------ (Estius)
— Against the spirits of wickedness: or wicked spirits in the air, says St. Jerome. Literally, in celestials. (Witham) — High places. That is to say, in the air, the lowest of the celestial regions in which God permits these wicked spirits or fallen angels to wander. (Challoner)