Last Wednesday, at the same hour, he was sitting on his Zen cushion meditating in the Claremont Avenue apartment he occupies as the Paul Tillich Professor of Theology, World Religions and Culture at Union Theological Seminary in New York.

A few hours later he was talking about his pointedly titled new book, “Without Buddha I Could Not Be a Christian” (Oneworld). The book is the outcome of decades of encounters with Buddhism — and of struggles with his own faith.

Born in 1939, Mr. Knitter began his path to the Catholic priesthood at age 13, studied theology in Rome during the years of the Second Vatican Council, was ordained in 1966, completed a doctorate in Germany and began a long and influential career as a scholar addressing questions of the relationship between Christianity and other world religions.

“Am I still a Christian?” he asks in his new book. It is a question posed over the years by others, including some unhappy officials in the Vatican. But the question, he writes, is also “one I have felt in my own mind and heart.”

“Has my dialogue with Buddhism made me a Buddhist Christian?” he writes. “Or a Christian Buddhist? Am I a Christian who has understood his own identity more deeply with the help of Buddhism? Or have I become a Buddhist who still retains a stock of Christian leftovers.”

The struggles Mr. Knitter is writing about are not the familiar ones about sexual ethics, the role of women or the failures of church leaders.

His focus here is on what he calls “the big stuff”: What does it really mean for Christians to profess belief in an almighty “God the Father” personally active in the world, or in Jesus, “his only-begotten Son” who saved humanity through his death and bodily resurrection, or in eternal life, heaven and hell?

However much he tried, Mr. Knitter found that certain longstanding Christian formulations of faith “just didn’t make sense”: God as a person separate from creation and intervening in it as an external agent; individualized life after death for all and eternal punishment for some; Jesus as God’s “only Son” and the only savior of humankind; prayers that ask God to favor some people over others.

Mr. Knitter’s response, based on his long interaction with Buddhist teachers, was to “pass over” to Buddhism’s approach to each of these problems and then “pass back” to Christian tradition to see if he could retrieve or re-imagine aspects of it with this “Buddhist flashlight.”

He was not asserting, as some people have, that religions like Christianity and Buddhism are merely superficially different expressions of one underlying faith.

On the contrary, he insists they differ profoundly. Yet “Buddhism has helped me take another and deeper look at what I believe as a Christian,” he writes. “Many of the words that I had repeated or read throughout my life started to glow with new meaning.”

Those new meanings will unsettle many Christians, as Mr. Knitter recognizes, even as they address difficulties felt by many others. This will vary, of course, from issue to issue. Mr. Knitter’s translation of Buddhist meditation into a call for a Christian “sacrament of silence” may be readily welcomed. His search for a “non-dualistic” understanding of God and the world may be only leading him through Buddhism back to Thomas Aquinas.

“Perhaps I could have come onto these insights without Buddhism,” he said Wednesday. Yet even in those cases he often expresses these insights in language that will be debated, like God as “InterBeing” or “Connecting Spirit.”

When his comparison between “Jesus the Christ and Gautama the Buddha” leads him to conclude that both are “unique” saviors but not sole or final ones, he is treading, as he well knows, in a theological minefield.

One can predict that this book will receive instant condemnation from people who feel their duty is to protect Christian doctrine from wandering off course.

One can also predict that those condemnations will, in turn, make others hesitant to voice more nuanced, thoughtful criticism out of fear of piling on.

Mr. Knitter and his book deserve better. It is easy to draw up a list of substantial criticisms. For one thing, Mr. Knitter’s Christianity comes laden with all the impurities of popular piety and workaday theology while his Buddhism seems to be that of the best and the brightest.

Some readers may detect the reflex of the lifelong recovering cleric in his recoiling from whatever might appear to be patriarchal or excluding. And most important are questions about the nature and use of religious language for pointing to a mystery that can never be captured in human words.

Yet serious critics, no matter how major their differences, will not be able to ignore the enormous, almost disarming honesty of this book. Mr. Knitter admits his painful puzzlements and conducts his search for answers out in the open. He does not hide behind academic abstraction but writes clearly and personally and leaves himself open to correction.

Although he argues for a kind of religious “double-belonging,” he does not hesitate to ask whether this is ultimately a kind of promiscuity — or, as one of his students put it, “spiritual sleeping around.”

Mr. Knitter doesn’t believe so. But he has written his book in part to see whether fellow Christians agree.

Will his “double-belonging” resonate sufficiently within his own faith community that he can continue to consider himself a Buddhist Christian? Or if not, as he explained this week, will he feel obliged to recognize himself as a Christian Buddhist?

One need not have a stake in that outcome to find “Without Buddha I Could Not Be a Christian” a compelling example of religious inquiry.