For us to grow in sanctity, while it is important to grow in the virtues and stay faithful to our prayers, we must also be on guard not to adopt the errors of the secular world. There are many "wolves in sheep's clothing." This is why we must strengthen our Faith, not only by learning its sacred doctrines, but avoiding the heresies and errors opposed to it. The Athanasian Creed (often called Quaecumque from the first word in the Latin) was recited by all clergy every single Sunday.
Keep the Faith!
Whoever wishes to be saved, needs above all to hold the Catholic faith; unless each one preserves this whole and inviolate, he will without a doubt perish in eternity. But the Catholic faith is this, that we venerate one God in the Trinity, and the Trinity in oneness; neither confounding the persons, nor dividing the substance; for there is one person of the Father, another of the Son, (and) another of the Holy Spirit; but the divine nature of the Father and of the Son and of the Holy Spirit is one, their glory is equal, their majesty is coeternal. Of such a nature as the Father is, so is the Son, so (also) is the Holy Spirit; the Father is uncreated, the Son is uncreated, (and) the Holy Spirit is uncreated; the Father is immense, the Son is immense, (and) the Holy Spirit is immense; the Father is eternal, the Son is eternal, (and) the Holy Spirit is eternal: and nevertheless there are not three eternals, but one eternal; just as there are not three uncreated beings, nor three infinite beings, but one uncreated, and one infinite; similarly the Father is omnipotent, the Son is omnipotent, (and) the Holy Spirit is omnipotent: and yet there are not three omnipotents, but one omnipotent; thus the Father is God, the Son is God, (and) the Holy Spirit is God; and nevertheless there are not three gods, but there is one God; so the Father is Lord, the Son is Lord, (and) the Holy Spirit is Lord: and yet there are not three lords, but there is one Lord; because just as we are compelled by Christian truth to confess singly each one person as God and [and also] Lord, so we are forbidden by the Catholic religion to say there are three gods or lords. The Father was not made nor created nor begotten by anyone. The Son is from the Father alone, not made nor created, but begotten. The Holy Spirit is from the Father and the Son, not made nor created nor begotten, but proceeding. There is therefore one Father, not three Fathers; one Son, not three Sons; one Holy Spirit, not three Holy Spirits; and in this Trinity there is nothing first or later, nothing greater or less, but all three persons are coeternal and coequal with one another, so that in every respect, as has already been said above, both unity in Trinity, and Trinity in unity must be venerated. Therefore let him who wishes to be saved, think thus concerning the Trinity. But it is necessary for eternal salvation that he faithfully believe also the incarnation of our Lord Jesus Christ. Accordingly it is the right faith, that we believe and confess, that our Lord Jesus Christ, the Son of God is God and man. He is God begotten of the substance of the Father before time, and he is man born of the substance of his mother in time: perfect God, perfect man, consisting of a rational soul and a human body, equal to the Father according to his Godhead, less than the Father according to humanity. Although he is God and man, yet he is not two, but he is one Christ; one, however, not by the conversion of the Divinity into a human body, but by the assumption of humanity in the Godhead; one absolutely not by confusion of substance, but by unity of person. For just as the rational soul and body are one man, so God and man are one Christ. He suffered for our salvation, descended into hell, on the third day arose again from the dead, ascended to heaven, sits at the right hand of God the Father almighty; thence he shall come to judge the living and the dead; .at his coming all men have to arise again with their bodies and will render an account of their own deeds: and those who have done good, will go into life everlasting, but those who have done evil, into eternal fire. This is the Catholic faith; unless every one believes this faithfully and firmly, he cannot be saved.
This is clear. Thank you. God bless you.