When I was younger, my mom used to sew. She made a lot of my clothes when I was growing up. Anyway, we had quite a few pair of scissors in our house, and we used them for many things: cutting paper, string, plastic, whatever. But mom's good sewing scissors were off-limits. They were the scissors she used only for cutting material. Once in a while, my brothers or I might need a pair of scissors in a hurry, and mom's sewing scissors were the first ones we found, then we used them for the paper, plastic, whatever. Unfortunately, when she went to use them for cutting material, she knew instantly that they had been used for something else that she never intended.
In the same way, we've been brought back to God to be used for His purpose and glory, not for ours or the world's. Unfortunately, we get ourselves in situations where we give in to ourselves or situations around us, and we are no longer the pure holy vessels that God intends us to be.