The very first question in the Bible came from the mouth of Satan, and it had only one purpose: to sow the seeds of doubt in the minds of Adam and Eve and make them disbelieve what God had told them (see Genesis 3:1-4). It's no wonder Jesus called the devil "the father of lies" (John 8:44).
Does this mean the devil is directly responsible for every doubt we have? Not necessarily; often doubts come from within our own hearts and minds. But in reality it doesn't matter where our doubts come from; the important thing is what we do with them.
Some people, for example, savor their doubts and enjoy having them (although they may not even realize it). They aren't really interested in finding answers to their questions or having their faith grow; their doubts are simply an excuse to keep away from God. But others grow through their doubts. They take seriously Jesus' promise to His disciples: "Seek and you will find" (Luke 11:9).
How should you deal with your doubts? First, face them honestly—and then commit them to God. Then saturate your mind and heart with the truth of God's Word, the Bible. I often say that the seed of faith grows best in the soil of God's Word. In addition, seek answers to your questions from wise and godly people.