Unforgivable Sin

"It is a sin against the Holy Spirit"

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.

"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."

Fr Paul Sawabe Takuma 沢辺琢磨 Father of Orthodoxy in Japan

Fr. Pavel Sawabe Takuma,

the one of the first three Orthodox Christian in Japan

The son of a samurai and son-in-law of a Shinto priest, Takama Sawabe was a fierce Japanese nationalist. He hated Christianity and all foreign influences in his country. One day he angrily confronted the Orthodox Christian missionary to Japan, a Russian priest-monk named Nicholas (Nicolai). Father Nicholas spoke to him:

“Why are you angry at me?” Fr. Nicholas asked Sawabe.
“All you foreigners must die. You have come here to spy on our country and even worse, you are harming Japan with your preaching,” answered Sawabe.
“But do you know what I preach?”
“No, I don’t,” he answered.
“Then how can you judge, much less condemn something you know nothing about? Is it just to defame something you do not know? First listen to me, and then judge. If what you hear is bad, then throw us out.”

After listening to Father Nicholas and learning about the Orthodox Christian way of life, the nationalist samurai who had once endorsed Shintoism now believed in Jesus Christ and was baptized, becoming the first person to embrace Orthodox Christianity in Japan. At his baptism, he appropriately received the Christian name Paul, after St. Paul, one of the Church’s greatest Apostles who, before his conversion, had used his authority to violently persecute the Christian Church. Paul Sawabe would eventually be ordained an Orthodox Christian priest.

Father Nicholas, the missionary who taught Paul the Orthodox Christian Faith and baptized him, was later consecrated as bishop and is today known as St. Nicholas of Japan.