Is this blasphemy against the Holy Spirit?

I once had the thought that maybe miracles, apparitions, and “saints” in the Eastern Orthodox Church (and all non-Catholic religions, for that matter) are possibly the devil’s deceptions to keep people away from the Catholic Church. My reasoning is that Satan can sometimes appear as an angel of light to confirm them in their errors. On the other hand, is this also blasphemy against the Holy Spirit? Is this an unpardonable sin? If not, do I have to confess it?

There is NO sin that cannot be forgiven - if there is repentance.

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