Then if a religious monument is speech, he's suppressing free speech. He explicitly said no other religious type monuments could be placed there, as his was "from God" and the others weren't.
And that is his opinion. But it does not establish a state religion because it is there. If you put a Montana state flag in an Indiana courtroom, it doesn't change the state. The monument did not establish a religion. It's merely a common philosophy. Some may revere it. Some may not. It's only a problem if people are forced to worship it.
Please source this. I have read his dedication speech and saw nothing of the sort and the only other objects that he refused were an atom, requested by an atheist group and the MLK Jr "I Have a Dream" speech according to the court opinion. The court also acknowledged that the rotunda was not a public forum in which any group could place objects of their choosing.