Why would an African American slave voluntarily join the armed forces

In many cases, slaves were promised freedom in exchange for service.

Not all. They are not common to both.

Judeo Christian's belief

An immortal soul

An afterlife for souls

One God

God is subject to fate or destiny

The world is a living thing, with body and soul

The earth began out of darkness and nothingness.

The God who creates the earth remains the ruler of all.

A great flood destroys most of humankind

A man builds an ark and is saved from the flood.

Greco Roman

God is void of nothingness.

What is common to both Judeo Christian and Greco Roman is

Concept of a Supreme Law or Ultimate Reality

Explanation:

Greco Roman and Judeo Christian religion differ from each other base on their belief. The Judeo Christian believed that only one God is the controller and ruled the universe while Greco Roman believe and accepts more than one god. They belief in many gods and one prayed and sacrifice to many gods. Though both religion beloved in supreme law but Judeo Christian believed in God's law and Greco Roman beloved in human written law.

God is a void of nothingness is not common to both. The Judeo Christian belief God is not empty, He is very much alive and full of alot of things.