"Atheism indicates the rejection of the idea of one God"

This is the view held by Christians and those of the branch of religions that stem from the 'Abrahamic Faith Systems', and of course any other of the other groups that also believe in Monotheism, including Zoroastrians, follows of Aten, The Flying Spaghetti Monster, etc.

Atheism just rejects the idea of the existence of any magical invisible sky fairy beings (at all, at all, at all!) with any powers outside those able to be exercised by anyone who understands how those powers work - there is no 'being outside the system', no 'miracles (things that cannot be explained inside the system)' - it has nothing to do with any number of gods in any pantheon, thus logically it also cannot accept just One of them either! :-)