His mother was Umayma d.'Abdu'l Muttalib. One of the four monotheistic hanifs mentioned by Ibn Ishaq. The others being Waraqa b. Naufal, `Uthman b. al-Huwayrith and Zaid b. Amr.
Ubaidullah became a Muslim and married Umm Habiba, daughter of Abu Sufyan. Both he and his wife joined the first band of persecuted Muslims who sought refuge under the Christian King of Ethiopia (Abyssinia) in 615AD. Some time after coming to Ethiopia, he gave up Islam and became a Christian, and testified his new faith to the other Muslim refugees. Ubaidullah lived and died a Christian in Ethiopia. Muhammad married his widow. Ibn Ishaq relates:
"`Ubaydullah went on searching until Islam came; then he migrated with the Muslims to Abyssinia taking with him his wife who was a Muslim, Umm Habiba, d. Abu Sufyan. When he arrived there he adopted Christianity, parted from Islam, and died a Christian in Abyssinia.Muhammad b. Ja`far b. al-Zubayr told me that when he had become a Christian `Ubaydullah as he passed the prophet's companions who were there used to say: ‘We see clearly, but your eyes are only half open,’ i.e. ‘We see, but you are only trying to see and cannot see yet.’ He used the word sa'sa' because when a puppy tries to open its eyes to see, it only half sees. The other faqqaha means to open the eyes. After his death the apostle married his widow Umm Habiba." (Ibn Ishaq, The Life of Muhammad, tr. Guillaume, 1967, p. 99)
Further reading: Testimonies of former Muslims