The evil spirits exist and are very active - Pray and use the Sacramentals

St. Teresa of Avila had quite alot of experiences with the forces of evil and here are some writings from her on her experiences:

10. May what was said be of help that the true servant of God might pay no attention to the scarecrows the devils set up in order to cause fear. We should know that each time we pay no attention to them they are weakened and the soul gains much more mastery. Some great benefit always remains, which I won't go into so as not to enlarge. I shall only mention what happened to me on the night of All Souls: while I was in the oratory after having recited a nocturn and while saying some very devotional prayers that come at the end, a devil appeared on the book so that I couldn't finish the prayer. I blessed myself, and he went away. When I began again to recite the prayers, he returned. I believe it was three times I began, and until I threw holy water at him I couldn't finish. I saw that some souls left purgatory at that instant; little must have been lacking to their freedom, and I wondered if he had aimed at preventing this.

A few times I've seen him in physical form, but many times with no physical form-as for instance in the vision mentioned above in which without seeing any form one knows he is there.

11. I also want to tell the following because it frightened me a lot: one day on the feast of the Trinity, being in the choir of a certain monastery and in rapture, I saw a great battle of devils against angels. I couldn't understand what that vision meant. In less than fifteen days it became easily understandable on account of a certain conflict that arose between people of prayer and many who were not, and a lot of harm was done in the house in which it took place. It was a battle that lasted a long time and caused much disquiet.

At other times I saw a large multitude of devils around me, and it seemed that a great brightness encircled me, and this prevented them from reaching me. I understood that God was watching over me so that they could not get to me in order to make me offend Him. From what I sometimes saw in myself, I understood that it was a true vision. The fact is that now I have understood so well the little bit of power he has provided I'm not against God, that I have almost no fear . The powers of devils are nothing if these devils do not find souls cowardly and surrendered to them: it is with such souls that they show their power. Sometime, in the temptations I already mentioned, it seemed to me that all the vanities and weaknesses of the past were again awakening within me; I had really to commend myself to God. At once the torment came of thinking that since those thoughts arose in me the favors I experienced must all be from the devil. It seemed to me that there shouldn't have been even the first stirrings of a bad thought in one who was receiving so many favors from the Lord. But then my confessor put me at peace.

*Above she notes that each time we pay no attention to 'them' meaning the evil spirits we give them less power.  Today I would say there are some in deliverance ministries especially here in Ireland who give evil spirits too much attention and this is dangerous.  If a deliverance ministry is not properly guided by the authority of the Church then stay away from it.  Deliverance ministries are not for everyone and certainly not for those who suffer with emotional and psychological instability.  Always refer to the teachings of the Church and writings of the Saints on these matters and leave everything to God.  

Pray much and for others, go and receive the Sacraments regularly and live an honest and virtuous life practicing the corporal and spiritual works of mercy and give thanks to God in everything.