"No service offered to God is ever wasted."
I first encountered this saying in Elisabeth Elliot's Keep a Quiet Heart years ago. Not only did I think it a good saying, but I often claimed it when I was down in the dumps about the lack of success I'd had in writing. I think I even quoted it once in an interview.
Lately God's been convicting me that it's not true. For the most blatant example, take Cain, who offered the fruit of his labors to God in all sincerity and, I'm sure, complete confidence that God would be as pleased as Cain was by all his hard work (much more than Abel had put out). We know that because the Bible tells us how mad Cain got when God didn't accept what he'd offered.
But we also know that God is fair. He had to have provided instruction for the kind of offering He wanted before the fact, which is why He rebuked Cain for his anger afterward. In fact we know that Adam had already been shown the need for a blood sacrifice, since an animal had to die to provide skins for himself and his wife to cover their nakedness in the Garden after the Fall.
It's hard to imagine Cain did not hear the instruction, whether it was given by God directly or by his father Adam. But hearing doesn't always mean receiving. Who knows what he was thinking when he "heard." Perhaps he wasn't really listening, or didn't think it mattered. Perhaps zeal (arrogance? competition with Abel?) prompted him to want to do even more than what God asked. In fact it must've seemed to Cain completely unacceptable to have to go to his younger brother to get the lamb for sacrifice. Then he would have had to do nothing, since Abel had done all the work of keeping the sheep. How much better, his human mind must have reasoned, to give of his own labor. Surely that would be ever so much more meaningful to God -- all Cain's blood, sweat, tears and effort through the months of tending his crops. Now there was a sacrifice! A real giving of himself.
Unfortunately God wanted nothing to do with it. Thus all of Cain's service, though offered sincerely, was wasted effort.
As with his, so can our efforts be wasted if our good deeds are done in the power of the flesh rather than the Spirit. (I Cor 3:12-15) And anything done in violation of God's word (instructions) cannot possibly be done in the power of the Spirit. Which is why it's so important that we know exactly what the Scripture teaches about what God wants of us in terms of service, rather than depending on our feelings or traditions or the opinions of others.