The practice of addressing God in the singular (rather than pantheistic plaural) is already found in the Egyptian Old Kingdom. I'm not sure whether the abstract deity was also by then referred to as "Father". I'd have to research that. Certainly this had occurred by the 18th Dynasty. See:
In a practical sense, the Great King was put forth as manifestation of "Father" and "God" in the world. When Jesus prayed "Father, take this cup from me", he no doubt was making an appeal to the royal family "God-Father", that is, to one who literally had the power to effect the crucifixion or provide a substitute. Of course, there has always been an instinctive urge of praying to the One, the unseen hand in the Universe, whether it actually exists or not.
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