So I cast "Heaven" in my main phase just to get "Earth" in my graveyard.
I tell my opponent who has an active Deathrite that I then cast "Earth", killing his Deathrite.
My opponent tells me, "You can't cast it from your graveyard because my Deathrite just ate it."
I tell him, "You can't respond to any triggers when "Heaven" resolves and I still have priority. I can cast "Earth" before your Deathrite can do anything."
It depends on whose turn it is. Since you said it is your turn, you are correct, though your reasoning is slightly flawed. When you cast Heaven, your opponent has a chance to respond while it is on the stack since you need to pass priority before it resolves. However, the spell is still on the stack in that case, so it is not in the graveyard to be targeted by Deathrite.
Then, when it resolves, the stack is empty and you get priority since it is your turn. This means you get the chance to take an action. As long as that action is casting Earth, it immediately goes to the stack as part of casting it so it again is not in the graveyard when your opponent gets priority. There are no triggers in your scenario so your explanation of "not responding to triggers" isn't accurate.
In fact, if you cast Heaven for X > 0 so it does kill a creature and then that creature dying does cause a trigger to occur (say, with Vicious Shadows or if you cast it with X = 0 but your opponent controls a Vulturous Zombie) it will mean that you will need to pass priority with the trigger on the stack before it resolved. This would give your opponent an opportunity to exile the Aftermath card since you can't cast a Sorcery (Earth in this case) while the stack is not empty.
Deathrite Shaman
So I cast "Heaven" in my main phase just to get "Earth" in my graveyard.
I tell my opponent who has an active Deathrite that I then cast "Earth", killing his Deathrite.
My opponent tells me, "You can't cast it from your graveyard because my Deathrite just ate it."
I tell him, "You can't respond to any triggers when "Heaven" resolves and I still have priority. I can cast "Earth" before your Deathrite can do anything."
What's the real ruling? thanx in advance...
Heaven // Earth
Deathrite SHaman
It depends on whose turn it is. Since you said it is your turn, you are correct, though your reasoning is slightly flawed. When you cast Heaven, your opponent has a chance to respond while it is on the stack since you need to pass priority before it resolves. However, the spell is still on the stack in that case, so it is not in the graveyard to be targeted by Deathrite.
Then, when it resolves, the stack is empty and you get priority since it is your turn. This means you get the chance to take an action. As long as that action is casting Earth, it immediately goes to the stack as part of casting it so it again is not in the graveyard when your opponent gets priority. There are no triggers in your scenario so your explanation of "not responding to triggers" isn't accurate.
In fact, if you cast Heaven for X > 0 so it does kill a creature and then that creature dying does cause a trigger to occur (say, with Vicious Shadows or if you cast it with X = 0 but your opponent controls a Vulturous Zombie) it will mean that you will need to pass priority with the trigger on the stack before it resolved. This would give your opponent an opportunity to exile the Aftermath card since you can't cast a Sorcery (Earth in this case) while the stack is not empty.