Let me share that I am very glad that in this forum my nerdiness for looking up the etimology of words in order to understand things better is more likely to find ressonance.
I would add that one can be grateful for without being grateful to. Religion teaches that a person should give thanks, but what is important is to feel thankful.
Aligned with your comment, lLet the nerdiness begin:
Adjective
grateful (comparative gratefuller or more grateful, superlative gratefullest or most grateful)
>>>Appreciative; thankful.
Adjective
thankful (comparative more thankful, superlative most thankful)
>>>Showing appreciation or gratitude.
Noun
thank (plural thanks)
>>>(obsolete) An expression of appreciation; a thought.
Etymology 1
From Middle English thank, from Old English þanc (“thought, favour, grace, pleasure, satisfaction, thanks”), from Proto-Germanic *þankaz (“thought, remembrance, gratitude”)...
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No object is mentioned. Once again, it seems like the word may have been repurposed.