The God of the Moon
Abraham lived in a major city called Ur of the Sumerian Kingdom in 2000 BC.
In the city Ur they revered the God of the Moon. Then Abraham, his father, and other family members moved to Harran, a border city of the Sumerian kingdom where the God of the Moon was also revered.
So, it is natural to think that at least some of the family members of Abraham revered the God of the Moon. But what was the Sumerian God of the Moon?
Sin or Nanna was the god of the moon in Mesopotamian mythology. Nanna is a Sumerian deity, the son of Enlil and Ninlil, and became identified with Semitic Sin. The two chief seats of Nanna's/Sin's worship were Ur in the south of Mesopotamia and Harran in the north...
Nanna's chief sanctuary at Ur was named E-gish-shir-gal ("house of the great light"). It was at Ur that the role of the En Priestess developed. This was an extremely powerful role held by a princess, most notably Enheduanna, daughter of King Sargon of Akkad, and was the primary cult role associated with the cult of Nanna/Sin.
Sin also had a sanctuary at Harran, named E-khul-khul ("house of joys"). The cult of the moon-god spread to other centers, so that temples to him are found in all the large cities of Babylonia and Assyria. A sanctuary for Sin with Syriac inscriptions invoking his name dating to the 2nd and 3rd centuries CE was found at Sumatar Harabesi in the Tektek mountains, not far from Harran and Edessa.According to the Sumerian myth, the God of the moon Sin had children who were the God of the Sun and the God of Venus.
http://en.wikipedia.org/wiki/Sin_(mythology)
Anyway the God in person spoke to Abraham, telling him to leave Harran heading for the place the God indicated. It means that Abraham left the God of the Moon, being guided by his own god. It is so since if Abrahan had continued to stay in Harran, he should have continued to revere the God of the Moon of Sumerians.
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Mar 3:28 Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme:
Mar 3:29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation.