It is not only taken for granted throughout the Bible but definitively taught by the Church that there exist, besides divine and human persons, other persons normally not visible in bodily form but able to act in our world—namely, good and bad angels (see DS 800/428, 3002/1783).
It is an important part of this teaching that these persons are created and wholly dependent upon God for their being; they have no independent reality over and against him. Nevertheless, although all are good insofar as they are creatures, some are bad insofar as by their own free choices they have determined themselves wrongly, and so exist in a certain disharmony with God, whose love would have shared with them his own perfect life (see DS 286/—, 325/—, 411/211, 797/427, 800/428).
It follows that the reality of the Devil and of demons cannot be denied; they are not merely mythical beings or the personification of evil. At the same time, the Devil must not be thought of as an absolute principle of evil opposed to the all-good God; there is no such principle. The Devil is in its entire positive reality good; its being remains relative to God and its evil limited by the scope of its own created (and still in itself good) freedom (see S.t., 1, q. 63, aa. 4–5; q. 64, a. 1).
There is more systematic perversity in the world than human sin accounts for. The demons account for this excess: “For we are not contending against flesh and blood, but against the principalities, against the powers, against the world rulers of this present darkness, against the spiritual hosts of wickedness in the heavenly places” (Eph 6.12). Vatican II teaches that the Devil had a role in the fall of Man and that its work helps to account for the monumental struggle which makes up human history (see GS 13; AG 9). Evil spirits still are at work in the world, seeking to damage the redemptive work (see 1 Pt 5.8; Rv 12.7–9). The Devil tries to lead Christians into sin (see 1 Cor 7.5; 2 Cor 2.11; 1 Thes 3.5; 1 Tm 3.7, and so on).
The Gospels present the redemptive life of Jesus as an encounter with and victorious struggle against the Devil. Jesus is tempted by the Devil, delivers many people from evil spirits, and ultimately overcomes the Devil by his passion and death (see Lk 22.3, 31, 53; Jn 13.2, 27; 14.30; Col 1.13). Hence, we are freed from the power of the Devil (see SC 6; cf. S.t., 3, q. 48, a. 4; q. 49, a. 2). No Christian can be conquered by the Devil without his or her own free choice to do what is evil (see Eph 4.27; Jas 4.7). The Church has firmly rejected tendencies to exaggerate the role of diabolical activity in temptation (see DS 2241–53/1261–73).
In accordance with an extensive theological tradition one can think of the sin of Satan as the first evil in creation, and so regard all other evil as somehow mysteriously related to it; in this view, one thinks of the diabolical realm as if it were a kind of perverse, wretched imitation of the kingdom Jesus is bringing to fulfillment (cf. 1 Jn 3.8–10). On this view, every temptation and sin can be credited to the work of the Devil (see S.t., 1, q. 114, aa. 2–3; 3, q. 8, a. 7). It instigates to sin and then seeks the sinner’s condemnation, so as to gain the lost soul for itself (see Rv 12.9–10).
Inasmuch as the power of the evil spirits has been broken by the victory of Jesus, one ought not to assume diabolical activity when other explanations are possible (see S.t., 1–2. q. 80). Still, in regard to temptation, those temptations which arise from one’s own prior sins are fittingly ascribed to the Devil, since by one’s sins one somehow surrenders oneself into the power of the Devil. Also, an awareness that there are evil spirits still at work in the world will inhibit anyone from too quickly assuming that every spiritual inspiration is a good gift of the Holy Spirit. One must put spirits to the test; those only can be trusted whose suggestions contain nothing incompatible with sound doctrine and moral truth (see 1 Jn 4.1–3).