“When I was a novice monk, for a certain period of time, the devil brought to me such blasphemous thoughts even when I was in Church, and I grieved over them a great deal. Whatever I had heard spoken by others, when I had been a soldier, swear words, curses and so forth, the devil would bring to my mind about the Saints.
“My Spiritual Father would say to me, ‘These thoughts are from the devil. The fact that a person is grieved over these impure thoughts which go through his mind about the holy and sacred things, this alone is already proof that they are not his own but, rather, come from outside.’ I, however, continued to be distressed by them…
“One day during the Divine Liturgy, at the Trisagion Hymn, I, with the other monks, was chanting quietly the Trisagion Hymn of Neleos. Then I saw a huge and fearful beast with a dog’s head entering from the door of the Litye. Flames were coming out of its mouth and its eyes! He turned and gave me two gestures of a curse, because I was chanting Holy God, Holy Mighty, Holy Immortal, have mercy upon us.
“I turned to the side to see if anyone else had seen the beast, but no one had. Later I told my Spiritual Father, ‘This and this happened to me.’ Then my Spiritual Father said to me, ‘There, you saw him. That’s him [who is the source of your blasphemous thoughts]. Now, will you stop worrying and be still.'”
St. Paisios of Mt. Athos (+1994) recounts the above story when counseling some nuns who were struggling with blasphemous thoughts. 
THE SOURCE OF BLASPHEMY
What are blasphemous thoughts? They are inappropriate thoughts or images about Christ, the Theotokos, the saints, icons, the Eucharist, our spiritual father, or anything else that is sacred or holy.
These thoughts originate from demons themselves and not from our own minds, though this may be impossible to perceive. With blasphemous thoughts, the demons will target people who are sensitive, as was the case with St. Paisios above. Demons will stream these thoughts through the person’s mind and then berate the person mercilessly for having such thoughts. It can make someone feel as if he is losing his mind.
I purposely did not name this blog “Struggling” or “Wrestling” with blasphemous thoughts, but rather “Rejecting” them. Why? Because these thoughts, unlike sinful passions, do not stem from the sinful corruption of our nature, but rather come straight from hell. They are not an unnatural part of us that we must struggle to uproot, but are like a demonic radio playing in our heads that must be kicked out the window.
CONFESSION OF THOUGHTS
It is important to note, however, our reaction to the blasphemous thoughts. If a person is grieved or troubled over them, then it is a sign that these thoughts have no home in him. However, if he finds some delight, amusement, or agreement with the thoughts, then he must immediately begin rejecting the thoughts and schedule a confession with his priest.
When in confession, we confess our sinful thoughts to the priest to receive forgiveness for them. However, it is not the same with blasphemous thoughts because these are the devil’s thoughts. If we were grieved by them, then we can simply say, “Father, I am having blasphemous thoughts about Christ (or anything else).” It is important that we let our spiritual father know this is going on so that he may pray for us. It can also help us feel a little less crazy. We also must not go into detail about what these thoughts entailed. Thoughts coming straight from hell are not healthy to ever recollect.
If someone has found some internal agreement with the blasphemous thoughts, it is important that he mention this in confession as well.
OVERCOMING THE THOUGHTS
The first and foremost thing to remember about blasphemous thoughts is that they are not our own. While feeling grief over them is natural, we must be careful not to slip into despair, depression, or even suicidal thoughts. St. Paisios recommended singing and chanting church hymns and psalms in order to combat blasphemous thoughts.
Singing such uplifting songs can prevent us from sinking into internal darkness. It also discourages the demon that is tormenting you. If he sees you praising God with all of your heart every time he gives you some blasphemous thought, he’s not going to want to keep doing such a thing for very long.
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.
If rejected immediately upon recognition, St. Paisios states that these will eventually go away. It may take weeks, months, or years, but we must not lose hope or ever feel ownership for these thoughts.
 The above material was taken from the chapter Blasphemous Thoughts from the book Spiritual Struggle, which is a book I highly recommend. It is published by the Holy Monastery of “Evangelist John the Theologian,” Souroti, Thessaloniki, Greece, 2010. A copy can be purchased here or here.