Cardinal Thomas Collins, Toronto's Roman Catholic archbishop, is urging Justin Trudeau to reverse his decision requiring new Liberal candidates to follow the party's "pro-choice" position.

Cardinal Collins wrote a letter to the Liberal leader on Wednesday saying there are two million members of his archdioceses.

"I encourage all of them, of whatever party, to serve the community not only by voting but by active engagement in political life as candidates. It is not right that they be excluded by any party for being faithful to their conscience," he wrote.

Trudeau announced the party's pro-choice policy on May 7. "I have made it clear that future candidates need to be completely understanding that they will be expected to vote pro-choice on any bills," Trudeau said.

The views of existing Liberal MPs, he said, would be "respected to a certain extent," but the party would not be open to candidates holding anti-abortion views.

The archbishop wrote in his letter than the party line should not trump matters of faith. "Political authority is not limitless: it does not extend to matters of conscience and religious faith. It does not govern all aspects of life."

Collins ended his letter with a direct challenge to Trudeau to stand down on his new rule.

"Political leaders in our day should not exclude such people of integrity, no matter how challenging they find their views," he wrote. "I urge you to reconsider your position"

Read the full letter here.