From The Holy Bible: Catholic Action Edition (1961) - Catholic Doctrine Guide:

AMERICANISM

The word became popular at the end of the past century in a movement that started with ideas of Father Hecker, the founder of the American Society of Paulist Missionaries. Americanism is a tendency rather than a system; it is very practical but lacks coherence. Pope Leo XIII knowing the danger, sent an apostolic letter to Cardinal Gibbons in 1889 and through him to the Bishops of the united States. In this letter the Pope brought out the main errors of Americanism ... 1) The contention that there is a need of adapting the Church to the demands of modern civilization and slant it towards a more democratic method 2) That there should be more scope for individual freedom of though and action since the Holy Spirit operates on the conscience of the individual more directly than on the hierarchy. The errors of Americanism take away from the importance of passive virtues such as mortification, obedience, contemplation, and concentration on the active virtues such as active apostolate and organization. After a thorough examination the Pope conclude with these words

“We cannot approve these opinions which constitute the so-called Americanism”.

Though acting with the highest motives, the “Americanists” find themselves eventually, as regards doctrine, in a position which is difficult to reconcile with the doctrine and tradition of the Church. Though the active life is very important it cannot be overlooked that Jesus Christ laid more stress on prayer and interior life; for on the internal life depends entirely the success of every Christian Apostolate.