The first reason I am not religious is because an Ultrapowerful Friendly Superintelligence (hereafter UFS) is the sort of thing that would have visible effects on the world, if it existed. Yet the predicted effects are not found.
Being fully benevolent, the UFS would naturally want to make the world better. It could best accomplish this goal by doing things. Yet there is a suspicious lack of activity attributable to the UFS. The religious will claim that, at some point in the future, the UFS will bring forth a wonderful world called Heaven or Paradise. There is no reason for this delay.
The second reason I am not religious is because the prevailing religions of our time, Christianity and Islam, both claim that God (whether called Yahweh or Allah) is precisely such a UFS, yet the behaviour they attribute to God is very far from the expected behaviour of a UFS.
Both religions claim that the UFS has repeatedly caused people to suffer. Yet the UFS would naturally prefer to accomplish its goals without such suffering, and it would be able to.
Both religions (in their non-watered-down forms) claim that the UFS is deeply annoyed when people fail to believe the Right Things, and that it therefore plans to torture the bulk of humanity for infinity years (or at least, to allow such torture to happen). This is highly implausible; the UFS would certainly not punish people for believing wrongly, and indeed would have little reason to care if people believed in it, aside from a desire that they know the truth.
In any case, the UFS could easily make us aware of its existence if it wanted to. The religious will claim that the UFS has announced its presence via the Bible or the Qur'an, but these books could easily be man-made (and indeed, at least one of them must be), so the UFS would not expect such books to be convincing.