Americanism in this context refers to a late nineteenth-century Catholic controversy, not to American Christianity or patriotism generally.
The controversy
European discussions attributed to some American Catholics a cluster of ideas: minimizing traditional doctrines to ease relations with modern culture, preferring active over contemplative virtues, diminishing religious vows, and giving excessive weight to individual initiative.
Testem benevolentiae
In 1899 Pope Leo XIII addressed the issue in the apostolic letter Testem benevolentiae to Cardinal James Gibbons. Gibbons and other American churchmen denied that the condemned system accurately represented their teaching. Historians therefore distinguish the propositions rejected by the letter from the broader and more complex experience of Catholicism in the United States.