You may have considered it polite and non-offensive but not everyone will, which is the point.
Replace unicorns with snake oil. The points stands. Belief lacking any factual basis that is protected by law may as well be the celestial teapot, pink unicorns or anything else without factual basis.
The point being is that in this case religion itself allows no flexibility. If you deny the holy spirit and evidently I do I cannot be forgiven ergo, according to this baseless belief I am a "sinner". Such a biased belief based on nothing at all should not have legal protection.
You see to deny the holy spirit is apparently a big deal and that offends people. Imagine how ridiculous it would be to deny the holy snake oil. Just as baseless, just as valid.
My argument is for sense. Legislation such as this has too much room for error. Queue "I'm offended" lawsuits.