What if an atheist commit the unforgivable sin, can he be forgiven if he converts?

Mark 3: 29, if one blasphemes the Holy Spirit then forgiveness is not possible. This is due to knowledge, 

There is no unforgivable sin. Jesus asked His Father in Heaven to forgive those who were nailing Him to the cross, while they were in the act of doing it. When we come to God, all our sins can be forgiven.

No.  An atheist can't commit the unforgivable sin.  By definition.  It is a complete and utter betrayal of Christ after having known for a fact that Christ is the son of God, our savior and redeemer from sin and from death.  Judas had to be a disciple of Christ in order to become a son of Perdition.  Atheists cannot become sons of Perdition by definition.

It isn't possible for any one to commit the unpardonable sin in this day and age and hasn't been since the end of the apostolic ministries.

To commit this sin firstly required a two-fold heavenly witness and the working of a miracle.

It was then that this sin could be committed by attributing the working of the miracles to have been performed by Satan and not the Holy Ghost (Holy Spirit). 

What part of "unforgivable" don't you understand?

And for those who apparently don't know, the unforgivable sin is blasphemy against the Holy Spirit.

What if an non pixie believer commit the unforgivable sin, and spoke against Pixie Spirit, can he be forgiven if he converts to Pixie God and Baby Pixie. The Pixie Trinity.

See what you are asking is the same.

Jesus said they won't be forgiven in the world to come which i take to mean when they're in heaven.

Jesus also said you would only be in >danger< of "hell fire" by this sin as he also said so for a few other sins. "Hell fire" i take to mean destruction or death and as Paul says we're saved from such a great death.

To blaspheme is to speak evil of Gods Holy Spirit. How can an atheist commit blasphemy if there is no God?

Hmmmm? 

Converts to what?

There is no unforgivable sin. And conversion is meaningless.