In the U.S. we have seen a foxhole ecumenism develop during the culture wars. Evangelical Protestants—historically the most anti-Catholic sector of the American Church—meet vibrantly faithful Catholics on the pro-life picket line, while Catholics realize that their best allies for upholding the definition of marriage happen to be Evangelicals. Old boundaries become permeable as theological differences get swallowed up in co-belligerency.
What happens at the picket line happens in seminaries and pastors’ studies. These days Protestant pastors read papal encyclicals for edification, and Western Christians discover unexpected wealth in the works of Orthodox liturgists. From the Catholic side, Vatican II, for all its excesses and false moves, has made the Catholic Church sound more Protestant because it has become more attuned to common biblical and patristic sources. Swimming the Tiber has become a popular Evangelical sport, partly because of the manifest attractions of Catholicism, partly because the Catholic Church is more hospitable to Evangelical concerns than anyone could have imagined in 1870 or 1950.
Source: Peter Leithart - First Things, "The Future of Protestantism: The Churches Must Die to be Raised Anew", August 2014, https://www.firstthings.com/article/2014/08/the-future-of-protestantism