Moses Maimonides, the great Jewish sage of the Middle Ages (also known as Rambam) viewed the elements of Genesis 26:5 as follows: "my charge" = belief in the unique divine Name and the oneness of God. Abraham safeguarded this belief and passed it on to others, thereby differing from the idol worshipers with which he was surrounded.

"My commandments" = the ones that God had given Abraham: to leave his home (Gen. 12:1), to sacrifice his son (Gen. 22:1), and to expel Hagar and her son (21:12).

"My ordinances" = to be gracious and merciful, to do justice and righteousness, and to command his children to do likewise.

"My teachings" = Circumcision and the Noachide commandments.

Another Jewish sage, Radak, also believed that the term "my decrees" referred to the Noachide commandments.

Other sages believed that Abraham was familiar with the entire Torah that God later gave Moses. As Maimonides points out, there is no evidence that this is the case. It is also clear that Isaac broke a number of the commandments that were later contained in Torah.

Laws of the Sons of Noah
The Jewish sages of Jesus’ day and beyond were much more familiar with the nuances of the TaNaKh (Old Testament) than are most modern Christian ministers myself included. They knew the text intimately, and had much of it committed to memory. After many centuries of pouring over the Hebrew text of Genesis, they concluded that prior to Moses’ day mankind was responsible to God for a universal code of conduct known as "The Seven Laws of Noah." In fact, to this day Jews believe that this code is the irreducible minimum required of all of Mankind. When a gentile seeks to fellowship with Jews in the synagogue, and yet not become a Jew, he is required to keep at least these laws. A. Cohen explains it this way:

The "practice of equity" meant having in place a justice system by which the other laws could be administered. "Immorality" refers to illicit sex of any kind. In reality, these were not individual laws, but law categories. Together they represented some 66 imperatives.

These seven mitzva’ot (commandments) were all derived from the first nine chapters of Genesis. Those who wish to research the specifics of how they were developed may consult Tractate Sanhedrin of the Talmud.

You will note that there is no category that includes tithing laws. Tithing never applied to the Gentiles unless they were living in Israel during the time of the Theocracy.

When Abraham is recognized for keeping God’s laws, it would be for those laws that were actually in force in his day – that is, the Noachide system. That system did not include tithing laws.

The Case of Abraham
Now let’s look closely at the single instance in which Abraham is said to have tithed. It is found in Genesis 14. The story picks up at the end of a successful military campaign in which Abraham and his men had rescued his nephew Lot and other family members from an enemy. Abraham had also plundered those who had committed the crime of kidnapping. When the campaign was over, "…he brought back all the goods…" (Genesis 14:16).

Melchizedek, king of Salem (later Jerusalem), threw a bread and wine party for Abraham and his men. This man was a priest (Cohen) of God. The priest then pronounced a blessing on Abraham (Genesis 14:19-20). In response, Abraham "gave him a tithe of all" (Genesis 14:20). This tithe (tenth) of the spoil was not a response to a law that was then in force. It was, as Dr. J.H. Hertz points out, merely a "thanksgiving offering" in recognition of the fact that Melchizedek was priest of the Most High God and it was God that had given him the victory.

Abraham’s act of tithing was entirely voluntary on his part, not obligatory.

Jacob’s Tithe Analyzed
Abraham’s grandson, Jacob, is also recorded as having offered God a tenth. The account is found in Genesis 28.

In the course of a journey, Jacob had made camp for the night. He used a stone for a pillow – upon which he probably placed a folded blanket – and fell asleep. As he slept, he dreamed. It was the now famous dream of "Jacob’s ladder" (Genesis 28:12 ff.).

In the morning, Jacob awoke and realized that the dream had been of supernatural origin. He had experienced a visitation from God. To commemorate the event, Jacob turned the stone he’d slept on, on its end, and then anointed it with oil. He named the location "House of God" – Bethel. Then Jacob made a vow:

"If God will be with me, and keep me in the this way that I am going, and give me bread to eat and clothing to put on, so that I come back to my father’s house in peace, then the Lord shall be my God. And this stone which I have set as a pillar shall be God’s house, and of all that You give me I will surely give a tenth to You" (Genesis 28:20-22).

Jacob, like his grandfather, was not acting in obedience to a law of tithing that was "on the books." Rather, his vow to tithe was part of an "if-then" formula – if God blessed and provided for Jacob, Jacob would then return a tenth of God’s provision to God. It was conditional. Incidentally, this is the first instance of a vow recorded in the Bible.

Conclusions
While voluntary tithing in the manner of Abraham and Jacob was an appropriate and thankful response to God’s blessing and provision, there is no reason to assume on the basis of these verses that there was in force a tithing law prior to Moses’ time.

Nor do these verses impose any obligation upon modern Christians to tithe to the Church. From the time of these two incidents, several centuries pass before tithing is again mentioned in Scripture. When it is, it is in the context of establishing the Aaronic priesthood and the Tabernacle service.