Theology (from the Greek Θεός meaning "God" and λόγος meaning "study of") is the systematic and rational study of religion and its influences and of the nature of religious truths, or the learned profession acquired by completing specialized training in religious studies, usually at a university or school of divinity or seminary.

Augustine of Hippo defined the Latin equivalent, theologia, as "reasoning or discussion concerning the Deity";Richard Hooker defined "theology" in English as "the science of things divine". The term can, however, be used for a variety of different disciplines or forms of discourse. Theologians use various forms of analysis and argument (philosophical, ethnographic, historical, spiritual and others) to help understand, explain, test, critique, defend or promote any of myriad religious topics. Theology might be undertaken to help the theologian:

Theology translates into English from the Greek theologia (θεολογία) which derived from theos (θεός), meaning God, and logia (λόγια), meaning utterances, sayings, or oracles (a word related to logos [λόγος], meaning word, discourse, account, or reasoning) which had passed into Latin as theologia and into French as théologie. The English equivalent "theology" (Theologie, Teologye) had evolved by 1362. The sense the word has in English depends in large part on the sense the Latin and Greek equivalents had acquired in Patristic and medieval Christian usage, though the English term has now spread beyond Christian contexts.

 Martin Luther (help·info) (10 November 1483 – 18 February 1546) was a German monk, priest, professor of theology and iconic figure of the Protestant Reformation. He strongly disputed the claim that freedom from God's punishment for sin could be purchased with money. He confronted indulgence salesman Johann Tetzel with his Ninety-Five Theses in 1517. His refusal to retract all of his writings at the demand of Pope Leo X in 1520 and the Holy Roman Emperor Charles V at the Diet of Worms in 1521 resulted in his excommunication by the pope and condemnation as an outlaw by the Emperor.

Luther taught that salvation is not earned by good deeds but received only as a free gift of God's grace through faith in Jesus Christ as redeemer from sin. His theology challenged the authority of the Pope of the Roman Catholic Church by teaching that the Bible is the only source of divinely revealed knowledge and opposed sacerdotalism by considering all baptized Christians to be a holy priesthood. Those who identify with Luther's teachings are called Lutherans.

Hamza Yusuf Hanson is an American Islamic scholar, and (with Zaid Shakir and Hatem Bazian) is co-founder of Zaytuna College in Berkeley, California, United States. He is a convert to Islam, and is one of the signatories of A Common Word Between Us and You, an open letter by Islamic scholars to Christian leaders, calling for peace and understanding. He has described the 9/11 attacks as "an act of 'mass murder, pure and simple'". Condemning the attacks, he has also stated "Islam was hijacked ... on that plane as an innocent victim".The Guardian newspaper in the United Kingdom reported that he "is arguably the west's most influential Islamic scholar" and added that "many Muslims find his views hard to stomach."

Hamza Yusuf was born to two academics in Washington State and raised in Northern California. In 1977, he became Muslim and subsequently traveled to the Muslim world and studied for ten years in the United Arab Emirates, Saudi Arabia, as well as North and West Africa. Hamza Yusuf spent four years studying in the United Arab Emirates and elsewhere in the Middle East. Later he traveled to West Africa and studied in Mauritania, Medina, Algeria, and Morocco under such scholars as Murabit al Haaj; Baya bin Salik, head of the Islamic court in Al-'Ain, United Arab Emirates; Muhammad Shaybani, Mufti of Abu Dhabi; Hamad al-Wali; and Muhammad al-Fatrati of Al Azhar University in Cairo, Egypt.^{[citation needed]} After more than a decade abroad, he returned to the United States and earned degrees in nursing from Imperial Valley College and religious studies at San José State University.^{[citation needed]}