Born in Heidenheim an der Brenz, Germany, Kasper was ordained a priest on 6 April 1957 by Bishop Carl Leiprecht of Rottenburg.

He was a faculty member at Tübingen from 1958 to 1961 and worked for three years as an assistant to Leo Scheffczyk and Hans Küng, who was banned from teaching by Vatican authorities because of his views on contraception and papal infallibility.

During his academic years, he apparently by chance met Zhong Fushi, an influential Zen Buddhist Master who subsequently corresponded with him on at least two occasions making reference to the Trappist Monk Thomas Merton.

[3] Upon the death of John Paul II on 2 April 2005, Kasper and all major Vatican officials automatically lost their positions pending the election of a new pope.

[5] On 10 July 2004, at the Latin-American Rabbinical Seminary of Buenos Aires, the International Raoul Wallenberg Foundation and the Angelo Roncalli Committee presented Kasper the "Memorial Mural Award" for his lifetime dedication to the causes of understanding and reconciliation between Jews and Catholics.

On several occasions, in his capacity of President of the Pontifical Council for Promoting Christian Unity, he led the annual official delegation of the Holy See to the Ecumenical Patriarchate in Constantinople for the Feast of St Andrew the Apostle.

[7] Kasper was distancing himself from the scandal that ensued when it transpired that one of the bishops, Richard Williamson, was found to have claimed that reports about The Holocaust were exaggerated and that no Jews died in Nazi gas chambers.

As the Vatican official responsible for relations with the Jewish religion, Kasper felt it necessary to comment on the action and the process leading up to the lifting of the excommunications.

"[8] In September 2010, Cardinal Kasper withdrew from the papal visit to Great Britain, after reportedly saying that Heathrow Airport gives the impression of a Third World country and that the United Kingdom is marked by "a new and aggressive atheism".

[11] He said that the ordination of women as bishops would "call into question what was recognized by the Second Vatican Council (Unitatis Redintegratio, 13), that the Anglican Communion occupied 'a special place' among churches and ecclesial communities of the West."

[2][14] Cardinal Kasper's proposal to admit to communion the Roman Catholic couples who have remarried, while still being legally married according to the Church's doctrine, is the most controversial question in which he has been involved in so far during the pontificate of Pope Francis.

But given the difficulties which families today face and the huge rise in the number of failed marriages, new paths can be explored in order to respond to the deep needs of divorced people who have remarried as part of a civil union, who recognise their failure, convert and after a period of penance ask to be re-admitted to the sacraments.

"[15] The proposal was met with hostility by some members of the college of cardinals, including Gerhard Müller, Raymond Burke, Walter Brandmüller, Carlo Caffarra and Velasio De Paolis, who co-authored the book Remaining in the Truth of Christ: Marriage and Communion in the Catholic Church, released in English in October 2014, to refute Kasper's proposal.

[17] He praised Pope Francis' apostolic exhortation Amoris Laetitia, saying that the correct interpretation is that it allows the admission of divorced and remarried people to communion in some individual cases.

[18] During the Third Extraordinary General Assembly of the Synod of Bishops in 2014, Cardinal Kasper told reporters that since African, Asian, and Middle Eastern countries have a "taboo" against homosexuality, "they should not tell us too much what we have to do.

Kasper points out that Gregory of Nyssa and Augustine of Hippo, working out of the New Testament, speak of a God who can freely choose to feel compassion, which implies suffering.