Moral theology is the study of a set of laws and moral codes laid down by a religious text, in order to govern the practitioners of that religion. This phrase most often refers to Roman Catholics, but Protestant Christians, Orthodox Christians, Muslims, and Jews also practice moral theology, though the laws are slightly different for each one. The doctrine of moral theology in each religion is meant to help the practitioners of that religion live upright, useful lives that will help them gain favor with God. Most of these laws have to do with interacting with those both inside and outside the religious community.
“How one is to act” is one of the primary phrases used in the study of Catholic moral theology. Priests study the Bible and other liturgical texts, often basing their homilies around how to live righteously. In the Catholic religion, this involves attending mass, taking communion, and following the Ten Commandments listed in the Bible. It also involves following various rules that may only apply to certain branches of Catholicism, or certain Catholic parishes. The idea is to keep from committing a mortal sin, which Catholics believe will keep a person out of Heaven for eternity.
Most Catholics believe that following the rules of moral theology will shorten their time in Purgatory, a place where the souls of the deceased go to find atonement for the sins they committed on Earth. The rules of Catholicism are meant to help keep practitioners pure so they can get to Heaven faster. Those that follow the rules only loosely are said to endure a longer period in Purgatory. Many of these rules and precepts also apply to Orthodox Christianity.
In Protestant Christianity, there is no Purgatory. Salvation, or accepting the Holy Spirit into one’s soul, is a guarantee that the acceptor will go to Heaven. For Protestants, moral theology is a kind of evangelism, showing others how Christians behave. The idea is that others will see this moral example and ask questions, causing the faith to spread far and wide. Protestants also believe that moral theology can help them feel more fulfilled by omitting vices and corrupting influences from their lives.
Some kinds of moral theology contain rules about diet. These rules may be relatively loose, such as the Catholic rule for not eating meat on the Fridays during Lent. Others may be very specific and somewhat strict. Muslims and Jews, for example, cannot eat pork. Jews must keep to a kosher diet, refraining from eating foods that are not approved as clean by Jewish law. Similarly, Muslims must eat a halal diet, which is similar — but not identical — to the Jewish kosher diet.