For an earnest young Christian named Ben Lowe, revelation came on the shores of Lake Tanganyika, in Africa. A relentless warming of the lake was reducing the catch of fish, the people were going hungry — and he had learned of scientific evidence that climate change was to blame.

For the Rev. Brian Sauder, who grew up attending a small Anabaptist church in rural Illinois, the moment came in a college classroom. Studying the fallout from environmental degradation, he learned of poor people who had to walk hours longer each day to gather firewood from depleted forests.

For both men, Christian duties that their upbringing had led them to regard as separate — taking care of the earth and taking care of the poor — merged into a morally urgent problem. “Why haven’t I ever made this connection before?” Mr. Sauder recalled asking himself.

It is a connection that many people of faith all over the world are starting to make.

The sweeping pastoral letter issued by Pope Francis on Thursday may prove to be a watershed, highlighting the issues of social justice at the heart of the environmental crisis. But the pope’s encyclical is, in a sense, simply an exclamation mark on a broad shift in thinking that has been underway for decades and extends far beyond the Roman Catholic Church.