'Blasphemy must be forbidden because it is against God'
Where is God? Is he real? Can you show me your God? Can you show me any evidence of your God? No, you cannot, and so this argument is meaningless to those who are not believers in your God.
'Blasphemy must be forbidden because it is against religion'
So you want to protect religion. Which religions will you protect? Do you want religious discrimination? If not, you must protect them all, both large and small.
Do you know how many religions there are? Do you know what strange beliefs they hold? Do you remember the people who thought that there was a UFO behind the comet? That was their religion!
So if you protect all religions against blasphemy, you will have to censor many, many truths - including the NASA information about the comet!
This situation is impossible because anybody can create a new religion at any time, and ask for new censorship. But if you refuse to listen to them, you would have religious discrimination.
'Blasphemy must be forbidden, when it offends many people'
Many religions are offensive to each other. For example, Islam says that Christ was a man, but Christianity"
.......... Freedom of Speech is under attack: