The philosophers who wished us to have the gods for our friends rank the friendship of the holy angels in the fourth circle of society, advancing now from the three circles of society on earth to the universe, and embracing heaven itself. And in this friendship we have indeed no fear that the angels will grieve us by their death or deterioration. But as we cannot mingle with them as familiarly as with men which itself is one of the grievances of this life, and as Satan, as we read, sometimes transforms himself into an angel of light, to tempt those whom it is necessary to discipline, or just to deceive, there is great need of God’s mercy to preserve us from making friends of demons in disguise, while we fancy we have good angels for our friends; for the astuteness and deceitfulness of these wicked spirits is equalled by their hurtfulness.XIX, 9

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