ROME — Pope Francis wrote a letter to a group of mostly South American migrants Thursday, calling them “the suffering flesh of Christ.” The pontiff told the 3,000 migrants gathered at Lajas Blancas in Panama en route to the United States that he would like to be there with them personally but instead was represented by a group of bishops from Costa Rica, Panama, and Colombia as well as Catholic pastoral workers. The migrants arrive in Lajas Blancas after crossing the Darien Gap, a dense tropical forest covering over 1,400,000 acres along the border between Panama and Colombia and one...