This was brought up I think about a week or two ago through the emails about be forgiven for sins. Here is one part of scripture that talks about the one sin that can't be forgiven.
Luke 12:10
[10] And whosoever shall speak a word against the Son of man, it shall be forgiven him: but unto him that blasphemeth against the Holy Ghost it shall not be forgiven.
[11] And when they bring you unto the synagogues, and unto magistrates, and powers, take ye no thought how or what thing ye shall answer, or what ye shall say:
[12] For the Holy Ghost shall teach you in the same hour what ye ought to say.
Hopefully this makes it a little more clear, that yes we can sin and we will sin. But we can be forgiven, EVEN those who possibly denied Jesus when he walked here on earth could have been forgiven if they repented. But if one blasphemes the Holy Spirit, he can never be forgiven. I think the main differences are the words "speak" and "blaspheme." Since Jesus, God, and the Holy Spirit are one, when a man simple speaks he is only speaking by man. But if one asks for the Holy Spirit to speak for them, and would go against it, then he is blaspheming the Holy Spirit by saying it is speaking when it is not.
Yours in Christ,
Stephanie