Check out this article if you're having blasphemous thoughts.
The article is basically a number of summaries of different articles, so you can pick the one that appeals to you the most.
Here is my favorite quote:
Remember, you're not crazy and this is not your sin. It is the devil's sin. His combative strategy in attacking us with blasphemous thoughts is twofold: (1) to see if we have some sort of agreement with them and, if so, to attempt to turn us into a little devil as well; (2) to try to drag us down into depression or hopelessness about our salvation because of the wicked thoughts going through our mind.
- Orthodox Road
Sums it up quite well.
Here is another good way of looking at it:
God lets his beloved be tempted because he believes in them. He trusts them to keep clinging to him no matter how strong and oppressive the temptation gets. Likewise, he doesn't prevent unwanted thoughts because he trusts each of us to keep believing in his love and saving power no matter how intense, hideous and distracting the thoughts get. And continually exercising such faith will end up making you spiritually strong, just like continually exerting physical effort would make you physically strong. Hold on: great things are ahead!
- Grantley Morris
We ought to bring every thought to the obedience of Christ.
Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ; And having in a readiness to revenge all disobedience, when your obedience is fulfilled.
- 2 Corinthians 10:5-6 (KJV)
God can help you gain the victory.
I can do all things through Christ which strengtheneth me.
- Philippians 4:13 (KJV)
Here's another good article about intrusive thoughts.
I also highly recommend you watch this YouTube video on how to fight the enemy.