Pope Leo XIII d 7/20 1903. He followed the long reign of Pius IX who largely defined the church in negative opposition to the modern world. Leo launched the era of Catholic social teaching. "I want to see the church so far forward that my successor will not be able to turn back."
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As was noted in one comment—Leo was followed by Pius X. Sometimes the journey of the church is like a game of Shoots and Ladders. At least the tradition of Catholic Social Teaching continued, though the next step forward had to wait 40 yrs after Leo’s Rerum Novarum.
Here I've been playing Snakes and Ladders all his time and getting nowhere.