If I were to tell you that I am the Son of God, what would you think of me? You'd think I was an idiot, a blasphemer, a devil and so one, right? And, you'd be right.
Jesus, during His earthly ministry, was man. He was mere flesh and blood. True, He was the manifestation of God in the flesh but He was, for all intents and purposes, a man. I think it was because He was man that He was willing to forgive those that spoke against Him out of ignorance. Being human, how could He expect people to believe Him to be anything but a human? For this reason He said this:
Jhn 10:37 If I do not the works of my Father, believe me not.
Jhn 10:38 But if I do, though ye believe not me, believe the works: that ye may know, and believe, that the Father [is] in me, and I in him.
Once His Message had been delivered and His Ministry fulfilled, He held people accountable to His Word. He had revealed Himself to them throughout His Life, Death and Resurrection- now, under the leadership and guidiance of the Holy Ghost to lead them to fulness of Truth, they were held accountable for their words and actions for or against the Holy Spirit. Once Jesus was completely and undeniably revealed as God manifested in the flesh by the Holy Ghost, any word spoken against the Holy Spirit would not be forgiven, for it is the Spirit of God that finally and completely reveals to us the divinity of Jesus.
Its like this:
A man dressed in "street clothes" walks up to me and says "You're under arrest". I look at him like hes a fruitcake and walk off. He grabs me by the arm and flashes His badge at me. I then recognize his authority. People did not recognize Jesus' authority until He fulfilled His Life, Death and Resurrection and the Holy Ghost was given. Jesus waited until He had completed His life, death and resurrection to say this:"ALL power (authority) in heaven and earth is given unto me..." He "flashed" His authority by giving us the inward dwelling Spirit of Christ to reveal all truth unto us.