When something is done or believed body and soul, it is done or believed completely. This term indicates great faith and commitment.
We all have bodies, and many people believe we have souls: a spiritual essence that lives after our bodies are gone. So if you think or act body and soul, you've brought everything you have to it. If you believe in a politician body and soul, you are 100% for that politician. If you work at your job body and soul, you are doing your job wholeheartedly. Many think of marriage as giving yourself body and soul to another person.