The answer to why we must mock religion is one I have touched on but never really answered directly. The answer is very simple, any ideology elevated above mockery becomes a tyranny.
Though ideology must never be above mockery, nothing is ever truly above mockery. Ridicule is the exception, ideology which is demonstrably harmful should be ridiculed, but other aspects of life should be off limits. Rule of thumb here, if it cannot be chosen then it's off limits unless it is demonstrably harmful.
The reason harmful ideologies must be ridiculed is to make them no longer acceptable, thus making them no longer harmful. Religion, as a whole, has been demonstrably harmful and is chosen. Prayer being an excellent example of the harm it causes to society.
Prayer has nothing to do with helping and everything to do with ego. People who pray are saying they are happy not actually doing anything but want everyone to think they are, so they announce that they are praying for someone who is suffering to draw attention to themselves, distracting from the person who is truly suffering.
We see this effect in medical doctors as well, the ideology has been acceptable for so long even the atheist doctors are willing to say platitudes in place of actually helping the patient, Virginia Mason and Harborview in Seattle do this quite a lot. Often prayer is used in place of real help resulting in the death of an innocent person, while the ones praying are allowed to continue to draw attention to themselves instead of being punished for their crimes.
Another event this problem often arises is when it takes very little to help, like say retweeting something on Twitter. Instead of tapping the retweet like they should, they just have to say "my prayers are with them." This makes it all about them and distracts from the actual problem. Some tweeps have actually gotten angry when I would point out how selfish they are being for saying such things.
To protect society from this selfishness and complacency, we mock prayer and religion, and ridicule the stupidity religion forces it's followers to embrace. Once it is no longer acceptable to harm others, or allow injustices to happen, because of religion, we will stop ridiculing and move on.
Now, before you get angry with any atheist who ridicules your religious nonsense, know this, we were once as stupid as you, and when we stopped being willfully ignorant we were ridiculed for wanting to learn, often abused for not accepting your myths, and distrusted for being more honest than any religious person has ever been. If you think silly pictures of your myths with phrases that show the idiotic ideas you believe are hurtful, at least we don't usually mock individuals until they display their stupidity.
We aren't attacking you, we are just telling the truth now.