In this very significant moment, Samson does not say my God,
or even use Yahweh.
He refers to the deity by the generic elohim.
He speaks to Delilah in language she would understand. For her part, this would not give her any reason to think of Samson as anything more than an ascetic devotee of any god.
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17 And he told her all his heart, and said to her, “A razor has never come upon my head, for I have been a Nazirite to God from my mother’s womb. If my head is shaved, then my strength will leave me, and I shall become weak and be like any other man.”