Knowing that someone or something is of the Holy Spirit, and saying it is from Satan, is blasphemy. The Bible states that all manner of sin shall be forgiven, but blasphemy of the H.S. shall not be forgiven. Yes, we can see the scripture in Mark, but also in Matthew 12:31,32 says it all. Verse 32 says whoever speaketh a word against the Son of man, it shall be forgiven him; but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come. These are not my words, but God's words. I believe it is a very serious thing to speak against the H.S. in any way, whether out of anger, or flippant words, or carelessness, etc. We are to be on our guard and watch all the time as Christians not to say any kind of blasphemous words, especially regarding the H.S. I cannot tell you if this is a sin that will not be forgiven, but look into God's word and He will guide you.