1-Most resources agree that both Cain and Able knew that there was a specific time set aside to worship with sacrifices being a part of said worship.I don't know everything Cain was thinking but since the sacrifice he chose to bring was lacking he must not have been all that grateful.
2-It differs not at all.Millions of people believe that God is real and if pressed will admit that He provides for their needs but they have no real relationship with Him.
3-I didn't give the offering it's name I only offered it as a point of conversation.
Do not people tithe every Sunday with a grudging spirit?I'd venture to say that if asked why they gave they might answer that it was in gratitude .

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