Is there such a thing as an unforgivable sin? Do not the Holy Scriptures teach us that sin against the Father and sin against the Son are forgivable, but not sin against the Holy Spirit?
How does one know if he/she has committed such a sin?
Ah, but how do you sin against the Holy Spirit? There is only one way, that is by consciously refusing to accept God's forgiveness. God does not forgive because someone refuses to accept His forgiveness. You cannot do that by accident. It takes a conscious and continuous effort on your part.
Archpriest Seraphim Slobodskoy, in chapter 34 of his book The Law of God, had this to say regarding the sinning against the Holy Spirit:
"However, whoever knowingly and persistently rejects the most saving mercy of God, which is a blessing of the Holy Spirit, and consciously calls the deeds of the omnipotent God the works of the Devil, he has no means of repentance, and without repentance there is not and can never be salvation."
Metropolitan Archbishop Sotirios of the Greek Orthodox Metropolis of Toronto puts it this way:
"The correct interpretation, as it is given to us by the Church Fathers, is this: blasphemy against the Holy Spirit is the denial by man out of hatred of God's power to save him. Even more simply, the man who does not believe that the grace of God--the Father, the Son, and the Holy Spirit--can save him, closes his heart to the actions of the Holy Spirit; he does not accept Grace. He does not proceed to repentance. He fights against the sanctifying and saving act of God. He creates within himself a sorrowful and incurable condition."
So in summary, it means that the deeds of the Holy Spirit are rejected and attributed to the evil one. Through this the person closes their heart to the action of the Holy Spirit, and therefore cannot be brought to that "godly sorrow" which "brings repentance that leads to salvation" (1 Cor. 7:10).
I hope you find this helpful.
Herman ~ American Carpatho-Russian Orthodox Diocese