Milton's Aeropagitica is considered one of the most important documents ever written against censorship. It calls for freedom of the press and religion. It is subtitled "A Speech" but was never given orally to Parliament, nor was this the intention. It was published and distributed in written form. In a nutshell, Milton argued that there were very few reasons to censor any writing:
In Athens where Books and Wits were ever busier then in any other part of Greece, I finde but only two sorts of writings which the Magistrate car'd to take notice of; those either blasphemous and Atheisticall, or Libellous.
Interestingly enough, these reasons form the basis of most anti-censorship laws in the United States today (libel, slander). The work also is quoted extensively. You may recognize:
As good almost kill a man as kill a good book: who kills a man kills a reasonable creature, God's image; but he who destroys a good book, kills reason itself, kills the image of God, as it were, in the eye.
That said, in order to discuss humanism in this work, one must know something about Milton himself. Humanism is a philosophy which puts man first in importance, before God. To answer this question, you are going to have to do some research into the bulk of Milton's writings and find out some information about Milton. Milton was a religous man. He would have put God first in importance, not man. Remember, this is the same man who wrote Paradise Lost. Even in Areopagitica, the arguments are based on what God's will would be.
So, in my opinion, although our humanistic society has borrowed much from this work to support freedom of the press, and we use these arguments to advance the idea that man is entitled to express himself freely, I don't believe this was Milton's intent. He may have believed that censorship was wrong, but if you read this work, you will see that he believes it is wrong only if it is libelous, slanderous or blasphemous.
You can read the text at the link below. There is a good analysis of the work right here on enotes.