In addition to Alikat’s comments, I would add the following:
From Bible Commentator Matthew Henry on 1 Sa 16:14
We have here Saul falling and David rising.
I. Here is Saul made a terror to himself (v. 14): The Spirit of the Lord departed from him. He having forsaken God and his duty, God, in a way of righteous judgment, withdrew from him those assistances of the good Spirit with which he was directed, animated, and encouraged in his government and wars. He lost all his good qualities. This was the effect of his rejecting God, and an evidence of his being rejected by him. Now God took his mercy from Saul (as it is expressed, 2 Sa. 7:15); for, when the Spirit of the Lord departs from us, all good goes. When men grieve and quench the Spirit, by wilful sin, he departs, and will not always strive. The consequence of this was that an evil spirit from God troubled him. Those that drive the good Spirit away do of course become prey to the evil spirit. If God and his grace do not rule us, sin and Satan will have possession of us. The devil, by the divine permission, troubled and terrified Saul, by means of the corrupt humours of his body and passions of his mind. He grew fretful, and peevish, and discontented, timorous and suspicious, ever and anon starting and trembling; he was sometimes, says Josephus, as if he had been choked or strangled, and a perfect demoniac by fits. This made him unfit for business, precipitate in his counsels, the contempt of his enemies, and a burden to all about him.<<<
I would also add that God is Sovereign and the times of our lives are truly in His hands. Satan and his demons cannot do anything on this earth, unless God allows it. If not for the restraining power of the Holy Spirit, Satan would probably destroy all those made in God’s image, or at the very least keep everyone in complete bondage to sin. Every one of us is either a child of the devil, or by the grace of God alone, an adopted child of God. God is not the author of evil, but He can remove His Hand of protection according to His sovereign will. Even then, satan and his demons can only do as much as God allows.
God allows things into our lives that we often don’t understand at the time they are happening. If we love God, however, we can rest assured that He is working all things for our good (Ro 8:28).
The greatest expression of faith in the Bible is found in Job 13:15
>>>"Though He slay me, yet will I trust Him" Job 13:15<<<
Paul, the Apostle, was ‘given’ a messenger from Satan:
>>>To keep me from becoming conceited because of these surpassingly great revelations, there was given me a thorn in my flesh, a messenger of Satan, to torment me. Three times I pleaded with the Lord to take it away from me. But he said to me, "My grace is sufficient for you, for my power is made perfect in weakness." Therefore I will boast all the more gladly about my weaknesses, so that Christ's power may rest on me. That is why, for Christ's sake, I delight in weaknesses, in insults, in hardships, in persecutions, in difficulties. For when I am weak, then I am strong. 2 Co 12:7—10<<<
Sometimes a sermon may be helpful, listen to the link below when you have some time: