Does the Catholic Church believe in the devil? I saw on television a priest who said this isn't official Catholic teaching.
The priest you saw on television, if he said what you say he said, is mistaken. Based on the teaching and example of Jesus (Mt 4:1-11; 12:22-30; Mk 1:34; Lk 10:18; 22:31; Jn 8:44), the Catholic Church has always held that the devil is real, not a mythical personification of evil. The Fourth Lateran Council (1215), in its decree condemning the Manichaean dualism of the Catharists, taught that "the devil and the other evil spirits were created good in nature, but they became evil by their own actions."
The Church's teaching on the subject is clear from its liturgy. At baptism, those to be baptized are called upon to reject Satan, his works, and his empty promises. The Church provides an official rite of exorcism, which presupposes, of course, the existence of Satan.
In 1975 the Sacred Congregation for Divine Worship issued a document called Christian Faith and Demonology. It explained the Church's teaching on the subject. This document quotes Pope Paul VI's teaching regarding the devil:
It is a departure from the picture provided by biblical and Church teaching to refuse to acknowledge the devil's existence; to regard him as . . . a conceptual and fanciful personification of the unknown causes of our misfortunes. . . . Exegetes and theologians should not be deaf to this warning.
Presumably this exhortation extends to priests who appear on television.
More recently, Pope John Paul II, in his general audience of August 13, 1986, expounded at length on the fall of the angels and, in speaking on the origin of Satan, said,
When, by an act of his own free will, he rejected the truth that he knew about God, Satan became the cosmic "liar and the father of lies" (Jn 8:44). For this reason, he lives in radical and irreversible denial of God and seeks to impose on creation--on the other beings created in the image of God and in particular on people--his own tragic "lie about the good" that is God.