Ecumenism or oecumenism mainly refers to initiatives aimed at greater Christian unity or cooperation. It is used predominantly by and with reference to Christian denominations and Christian Churches separated by doctrine, history, and practice. Within this particular context, the term ecumenism refers to the idea of a Christian unity in the literal meaning: that there should be a single Christian Church. Not to be confused with Nondenominational Christianity

The word contrasts with interfaith dialogue or interfaith pluralism aimed at unity or cooperation among diverse religions and referring to a worldwide 'religious unity' by the advocacy of a greater sense of shared spirituality.

The word is derived from Greek οἰκουμένη (oikoumene), which means "the whole inhabited world", and was historically used with specific reference to the Roman Empire. The ecumenical vision comprises both the search for the visible unity of the Church (Ephesians 4.3) and the 'whole inhabited earth' (Matthew 24.14) as the concern of all Christians.

Peter John Kreeft (born 1937) is a professor of philosophy at Boston College and The King's College. He is the author of numerous books as well as a popular writer of Christian philosophy, theology and apologetics. He also formulated, together with Ronald K. Tacelli, SJ, "Twenty Arguments for the Existence of God".

Kreeft took his A.B. at Calvin College (1959), and an M.A. at Fordham University (1961). In the same university he completed his doctoral studies in 1965. He briefly did post graduate studies at Yale University.

Kreeft has received several honors for achievements in philosophical reasoning. They include the following: Woodrow Wilson, Yale-Sterling Fellowship, Newman Alumni Scholarship, Danforth Asian Religions Fellowship, and Weathersfield Homeland Foundation Fellowship.

He joined the Philosophy faculty of the Department of Philosophy of Boston College in 1965. His intellectual reputation stems from his strengths in debating and summarizing the philosophical arguments of the major Western philosophers. He has debated several academics in issues related to God's existence. Shortly after he began teaching at Boston College he was challenged to a debate on the existence of God between himself and Paul Breines, an atheist history professor, which was attended by a majority of undergraduate students. Kreeft later used many of the arguments in this debate to create the Handbook of Christian Apologetics with then undergraduate student Ronald K. Tacelli, S.J..