Arguing against religious beliefs is expressing your own religious beliefs. If someone says "God loves you all" and I say "a loving God wouldn't let children suffer and die", We are both expressing our religious beliefs.
The only exceptions to freedom of speech are things like uttering threats, Hate speech, And libel (lying about someone in a way that hurts their interests). It's pretty easy to express disagreement with religious beliefs without doing any of those things.
There are some freedoms clearly spelled out and there should be some which can be deduced. So we have a Freedom of Speech and Freedom of Religion and there is Separation of Church and State -- I'd say we do have a Freedom From Religion, Which permits to write against it.
In the U. S. , We have the right to our own opinion and the right to voice that opinion. We also have the right to make claims against anything as long as we can validate those claims. Unfortunately, Today, What begins as opinion tends to progress into a claim. Hypothetically, A senator states, As an opinion, That they think a person is guilty of a crime. The media can take that opinion and state "According to this senator, That person is guilty" turning opinion into something that can easily be seen as a proclamation. This is just one of the ways the media tends to twist facts.
Thing is, It does not matter what topic it's about. The same freedom of speech should apply. This should also include opinions about race. Sadly, Some people want to declare any unpopular speech as "hate speech" even if it does not make any proclamation or if it's not intended to incite violence. Not that I agree with any racist beliefs but just that they should still have the same right to voice their opinion. If we allow one type of speech to be prohibited, It sets a precedent to outlaw any unpopular speech.
As Ettina said, You can say "a loving God wouldn't let children suffer and die". You are allowed to say that, But there are certain limits to expressing your feelings on religions. For example, You cannot discriminate people for their religious beliefs, Or religions. So this debate can come up to an agreement that there are certain limits to your rights.