Free speech is a Biblical virtue because to control one's speech is to control their mind. God has made all men, right? And He commands us to worship Him - to put our faith in His Son Jesus Christ for the forgiveness of sin. We know this (the gospel) because He's sent His prophets and preachers to the world with the message of reconciliation. The thing to note is that God gives all men not just life and liberty, but the right (for now...until Judgment Day) to disagree with Him. To blasphemy. To ignore Him. This is the the basis for freedom. This is the basis for free speech.
Learn so much more about this glorious truth in this podcast as well as why sin causes us to squelch debate when we can.