In the village of Ecully, under the guidance of the parish priest, whose assistant he had been appointed, he strove with all his strength to attain to the higher degrees of pastoral perfection. After three years had gone by, he was sent, like an Angel from heaven, to the small village of Ars, which not so long after was included in the diocese of Belley, and in a most brilliant manner he entirely renewed the condition of his neglected and forsaken parish. Continually engaged for many hours daily in hearing confessions and in giving spiritual direction, he introduced the frequent reception of the Eucharist, and organized pious sodalities: and in a remarkable manner he inspired into souls a tender devotion to the Immaculate Virgin. And, deeming that it is the duty of the pastor to expiate the sins of the flock accredited to him, he spared neither prayers, nor vigils, nor mortifications and continual fastings. Since Satan could not endure such great virtues in this man of God, he assailed him, first with mere annoyances, and afterwards in open combat; but John Mary patiently endured the most malevolent injuries.
He was very often asked by the neighbouring priests to labour for the salvation of souls after the manner of the Missionaries, either by preaching sermons, or by hearing confessions, and he was always at hand in every case. Burning with zeal for the glory of God, he brought it about, that the pious exercises of Missions were established in more than an hundred parishes arranged in a continuous and permanent series. Meanwhile, as God was rendering his servant famous by miracles and by graces, there began that celebrated pilgrimage, in which, throughout a period of twenty years, nearly one hundred thousand persons of every class flocked to Ars, not only from France and from Europe, but even from the distant regions of America. Worn out by labours rather than by old age, having foretold the day of his death, he went to rest in the embrace of the Lord, on the 4th day of August, in the year 1859, and of his age the seventy-third. After he became illustrious for many miracles, Pius X added him to the number of the Blessed, and Pius XI, in the holy year numbered him with the Saints in heaven and extended his feast to the universal Church, and on the fiftieth anniversary of his own priesthood, appointed him the heavenly patron of all parish priests.