On December 1st, he published a lengthy column titled "Sex and the Single Priest." Among other things, Keller tells us that he has not been a practicing Catholic since his high school days -- approximately 50 years ago. (Disclosure: For years now, I have not been a practicing Catholic.)
The gist of Keller's argument is that the Roman Catholic Church should bring an end to the custom of requiring priestly celibacy. In short, priests should be allowed to marry.
However, we should distinguish between diocesan priests and priests in religious orders. The rules and customs governing celibacy for diocesan priests are church rules and customs. So they could be changed by the church. But the rules and customs governing each religious order would be far more difficult to change. Pope Francis, for example, joined the Jesuit religious order and took a vow of chastity as a Jesuit years before he was ordained a priest. So I will refer here only to diocesan priests.
Now, by what process could the rules and customs about celibacy for diocesan priests be changed? Frankly, I don't know for sure what the process would have to be.
But Keller focuses on Pope Francis. Keller says, "There is one issue, however, where the internal politics, while difficult, are less difficult, where the case for reform is pressing, and where there are hints that Francis may be inclined to change. That is priestly celibacy."
For the sake of discussion, let's say that Pope Francis could lead a reform of the requirement of celibacy for diocesan priests, leaving aside what all the process of such reform might involve.
But I would point out that Pope John-Paul II reaffirmed the requirement of celibacy for diocesan priests. Indeed, he made celibacy sound heroic. Many diocesan priests today are referred to as JPII priests. Oftentimes, they see themselves as heroic cultural warriors resisting and fighting against the forces of secularism.