Evil spirits

“Now the Spirit of the Lord had departed from Saul, and an evil spirit from the Lord tormented him.” 1 Samuel 16:14
This verse poses a problem for many people. Does it mean that God sends evil spirits, as well as good spirits? Not at all. All of God’s ways are right and holy. He does not make bad things happen. However, He does allow bad things to happen, and in this case, He allowed an evil spirit to torment Saul. When the Holy Spirit is absent, people have no protection against these evil spirits. But when the Holy Spirit is present, we have protection against evil spirits. This is why there is no way a Christian can be demon-possessed.
Once we have the Holy Spirit, He will not be taken from us. He is a guarantee of our salvation (2 Cor. 1:22, 5:5, Eph. 1:13-14). We can have no fear of the Holy Spirit being taken from us (in contrast to those who lived in the Old Testament - see for example Ps. 51:11). We may restrict His power operating in our lives, but He will always be with us.
God is in control of everything in this world and is using everything to work out His purposes. He used the evil spirit in Saul’s life to form David’s character, for a time when he would be attacked by his own family. Even Satan is used by God, to tempt us so that by the power of the Spirit we can learn to resist him. We don’t see the big picture now. But from eternity looking back, we will.