Obviously not judging by the lyrics of east jesus nowhere
Is Green Day Christian?
Nope.
No. Green Day is alternative rock or what they call pop punk, but they are not christian.
Listen to the lyrics of their songs and the answer will come to you (and that answer is no)
The best way to answer that question is by explaining them as being anti-religion. Their beliefs are very clear if you listen to their lyrics. They feel religion is out dated and is of form of control.