A hard battle. . .

407 The doctrine of original sin, closely connected with that of redemption by Christ, provides lucid discernment of man's situation and activity in the world.
By our first parents' sin, the devil has acquired a certain domination over man, even though man remains free. Original sin entails captivity under the power of him who thenceforth had the power of death, that is, the devil. (Council of Trent (1546): DS 1511; cf. Hebrews 2:14) Ignorance of the fact that man has a wounded nature inclined to evil gives rise to serious errors in the areas of education, politics, social action (cf. John Paul II, Centesimus Annus 25) and morals.

408 The consequences of original sin and of all men's personal sins put the world as a whole in the sinful condition aptly described in St. John's expression, "the sin of the world". (John 1:29) This expression can also refer to the negative influence exerted on people by communal situations and social structures that are the fruit of men's sins. (cf. John Paul II, Reconciliatio et Paenitentia 16)

409 This dramatic situation of "the whole world [which] is in the power of the evil one" (1 John 5:19; cf. 1 Peter 5:8) makes man's life a battle:

The whole of man's history has been the story of dour combat with the powers of evil, stretching, so our Lord tells us, from the very dawn of history until the last day. Finding himself in the midst of the battlefield man has to struggle to do what is right, and it is at great cost to himself, and aided by God's grace, that he succeeds in achieving his own inner integrity.

Vatican II, Gaudium et spes 37 § 2

414 Satan or the devil and the other demons are fallen angels who have freely refused to serve God and his plan. Their choice against God is definitive. They try to associate man in their revolt against God.

1237 Since Baptism signifies liberation from sin and from its instigator the devil, one or more exorcisms are pronounced over the candidate. The celebrant then anoints him with the oil of catechumens, or lays his hands on him, and he explicitly renounces Satan. Thus prepared, he is able to confess the faith of the Church, to which he will be "entrusted" by Baptism.

2482 "A lie consists in speaking a falsehood with the intention of deceiving."
(St. Augustine, De mendacio 4,5:PL 40:491) The Lord denounces lying as the work of the devil: "You are of your father the devil, . . . there is no truth in him. When he lies, he speaks according to his own nature, for he is a liar and the father of lies." (John 8:44)