After that he returned to Boston and completed a fellowship in geriatric medicine with the Harvard Medical School Division on Aging, before becoming director of geriatric consultation services at Beth Israel.

Throughout his time at Harvard he studied for his master of divinity degree at Weston Jesuit School of Theology in Cambridge and was ordained in 1994. He professed solemn vows in 2005.

On Aug. 8 he addressed Cardinal Seán P. O'Malley and the priests of the Archdiocese of Boston at St. John's Seminary in Brighton on why the clergy must actively work to oppose the initiative.

"My presentation focused on ways in which the priests could help parishioners understand the upcoming ballot initiative to be able to work with the fears and concerns that many have that might lead them to look at assisted suicide as a good thing," he said.

Father Sheehan put the issue in the framework of Catholic culture and said the shifting values in healthcare over the years have increased the focus on keeping patients alive at all costs rather than understanding natural death in the context of spiritual life.

"There is a bottom line that we have the fifth commandment 'Though shalt not kill,' and the killing of innocent life is considered intrinsically evil, that is, it is always wrong. And so to take the life, or to provide the means for a person to kill himself is considered an intrinsically evil act, because it violates first the life of the person. Second, it is a larger assault against what it means for human dignity," Father Sheehan said.

"The third part is that it diminishes all of us when the lives of some are considered expendable and not worth continued living," he said.