As humans we tend to question things. It's our nature and one of the many ways God has made us wonderful and in His image. Sometimes though, we tend to question our faith or wonder if we are truly walking according to God's will and purpose. While it's spiritually healthy to periodically do a little soul searching self-check to make sure we are still on the right path, Satan also uses doubt to discourage us in our walk with God.
One example of this is in Matthew 14:24-33, the story of Peter walking on water to Jesus:
25 And in the fourth watch of the night Jesus went unto them, walking on the sea.
26 And when the disciples saw him walking on the sea, they were troubled, saying, It is a spirit; and they cried out for fear.
27 But straightway Jesus spake unto them, saying, Be of good cheer; it is I; be not afraid.
28 And Peter answered him and said, Lord, if it be thou, bid me come unto thee on the water.
29 And he said, Come. And when Peter was come down out of the ship, he walked on the water, to go to Jesus.
30 But when he saw the wind boisterous, he was afraid; and beginning to sink, he cried, saying, Lord, save me.
31 And immediately Jesus stretched forth his hand, and caught him, and said unto him, O thou of little faith, wherefore didst thou doubt?
32 And when they were come into the ship, the wind ceased.
33 Then they that were in the ship came and worshipped him, saying, Of a truth thou art the Son of God.
When Peter's faith was strong, he could do the most amazing things - Peter was able to literally walk on water! But as soon as his faith wavered because of the boisterous wind, Peter began to sink. Thankfully he cried out to Jesus to help him and because of his cry to the Lord, Jesus stretched forth his hand and saved Peter.
When Satan has us doubting our self-worth in God, or even doubting that God really exists, we sink and experience defeat. It doesn't take huge, mountain-sized faith either - Matthew 17:20 says that our faith need only be the size of a mustard seed. That is just a tiny amount of faith, but it's so necessary to walk with God. If you happen to be struggling with that, there is one thing to do: pray. Pray for God to open your eyes and give you the faith that you need.
We must also take control of our own minds, and be firm not to entertain ideas that could weaken our faith. If you give Satan an inch, he will move right in and bring his sleeping bag. Know your own mind - choose a side and stick with it. It's either God or defeat. There is no other option. Lucky for us, choosing to put our faith in God is definitely a winning choice.