Baltimore Catechism No. 4 (of 4)
AN ACT OF FAITH
O my God! I firmly believe that Thou art one God in three divine persons, Father, Son and Holy Ghost; I believe that Thy divine Son became man, and died for our sins, and that He will come to judge the living and the dead. I believe these and all the truths which the Holy Catholic Church teaches, because Thou hast revealed them, Who canst neither deceive nor be deceived.
An "act," i.e., a profession, of faith. The whole substance of the act of faith is contained in this: I believe all that God has revealed and the Catholic Church teaches. We might mention one by one all the truths God has revealed, i.e., made known to us, and all the truths the Catholic Church teaches as revealed by God. For example, we might say, I believe in the Holy Trinity, in the Incarnation of Our Lord in the Holy Eucharist, in the Immaculate Conception of the Blessed Virgin, in the infallibility of the Pope, and so on, till we write an act of faith twenty pages long, and yet it would all be contained in the words: I believe all God has revealed and the Catholic Church teaches. Hence we find in prayerbooks and catechisms acts of faith differing in length and words, but they are all the same in substance and have the same meaning. The act of faith in our Catechism gives a few of the chief truths revealed, that it may be neither too short nor too long, and that all may learn the same words.