The little church in Chicago's Loop was his 11th or 12th stop of a day that had begun 17 hours and three cities earlier.

The fatigue that had been accumulating in the week of days like this since he had left the Vatican for his American tour had turned his normally ruddy checks to gray. But he stood there that night in 1979 and made no effort to still the applause so that he and move on to two more stops he must make before he could finally get a few hours' rest.

Like teen-agers at a rock concert, the congregation of priests and members of the Christian Brothers order shouted and cheered and applauded and would not stop. But he did not chide them, only stood there before them, hands clasped over his white robes, and nodded in a gesture that seemed to say: "Yes, I am here. I am at your disposal. Use me. That is why I am here."

It was that same sense of mission, the willingness to be of service, to be used, that made Pope John Paul so vulnerable this week to that gunman in St. Peter's Square.

Much has been made of Pope John Paul's passion to work the crowds, to kiss babies, to press the flesh. To those watching him do just that, it has been immediately evident that he revels in such experiences, indeed seems to draw a kind of nourishment from them.

It is probably inevitable that one of the first reductions to the assassination attempt, coming as it did only a few weeks after the attempt on President Reagan's life, was to counsel better security and a cutback on the pope's close contact with crowds.

Those who are familiar with John Paul's outlook believe it is unlikely that such counsel will be heeded. As Bishop Thomas Kelly, general secretary of the National Conference of Catholic Bishops, observed, "It's harder to tell a pope what to do than a president." Kelly, who not only accompanied the pope during his U.S. tour but offered advice on the scheduling of it, speaks from experience.

More than any pope in recent history, John Paul has made his accessibility, his public presence, a hallmark of his papacy. Whether in St. Peter's Square, a street in Harlem or a Mexican highway, he has waded into crowds, touched the people, tried on their hats and talked with them in their own language.

That he has enjoyed these forays is beside the point, except that it has enabled him to accomplish his purpose more gracefully. The pope not only heads the Catholic Church but, in a sense, symbolizes and personifies it as well. When the pope plods through the mud of a Brazilian slum, he takes the church with him.

Wading into crowds all over the world, John Paul has taken the church to the people. Setting aside for the moment the arguments about doctrine and faith and morals, by his very presence he has symbolized the unity of the church and its accessibility. He has also simply been following the example set by Jesus, who, according to scriptual accounts, spent His time with the people.

John Paul inherited the papcy at a time of deep division in the Roman Catholic Church, with the many revolutionary changes mandated by the Second Vatican Council still only partly digested and regard for institutional religion generally at a low ebb. Most Catholics generally feel at least a vague loyalty toward any pope; the intensely personal style of Pope John Paul has focused and intensified that loyalty, as witness the outpouring of concern this week when he was attacked.

For some in the church, John Paul's re-emphasis on the church's ban on birth control and divorce, his opposition to opening the priesthood to women and married men, have been a disappointment. Others, who applaud those convictions, grow nervous when he criticizes sharply, as he did last year in his visit to Brazil, economic and political systems that leave "a big gap between a minority of the rich on the one hand, and the majority of those who live in want and misery, on the other."

But he is the pope of both liberal and conservative, resonsible for holding the church together. His 1979 tour of this country not only gave all Catholics a new pride in and a new identification with their church, but for at least a week, it focused as never before the attention of a generally secularized nation on a religious leader and the values and ideals he represented. To be sure, he was criticized for some of the positions he took, but that, too, is part of his vulnerability.

The pope is fond of quoting to his priests the New Testament statement that "the good shepherd is one who lays down his life for his sheep." He represents, after all, a religion whose founder did not turn away from that advice.

It may be that John Paul after this week's shocking assault, will heed the security experts, strengthen the Swiss guards, stay out of the crowds, and order a bullet-proof cassock.

But I think about him, standing before those cheering priests in St. Peter's Church in Chicago, unspeakably weary but still infinitely patient, offering himself to them, and I doubt it.