The First Cause? Prime Mover Argument

[Angrboda]

My experience of philosophy has taught me that every belief system has major problems with justifying itself in terms of its fundamental assumptions(including the scientific one), so what you believe is largely a choice.

This only matters when the fundamental assumptions diverge. Most often that is not the case. Things like realism tend to be shared assumptions between atheist, deist and theist. Did you have a specific assumption in mind?