The most important divine secret of the faith that Jesus Christ revealed to us is the mystery of the Most Holy Trinity. Jesus spoke of his Father, who is God; of the Holy Spirit, who is also God; and affirmed that He and the Father are one and the same thing (John 10:30), because He is the Son of God. The Father, the Son and the Holy Spirit are one God - not three gods - because they have the same divine nature, although they are three really different Persons.
That God is one in essence and triune (three) in persons is the revelation of his intimate life, the greatest and deepest of all mysteries; at the same time it is the fundamental mystery of our faith and of our Christian life. We have to try to know it and live it! The Creed or Symbol is the explanation of the Trinitarian mystery: what God is and what he has done for his creatures when creating them, redeeming them and sanctifying them.
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The mysteries we will never be able to understand them because we are limited and they surpass us; However, we must try to know them better and better, so that our faith is firm and operative.
The mystery of the Most Holy Trinity is that in God there is only one essence and three different persons: Father, Son and Holy Spirit, each of whom is God, without being three gods but one and only God.
We can compare this mystery with the sun: the sun is in the sky and produces light and heat; Light and heat are not different from the sun. For something similar is the Trinity: the Son and the Holy Spirit are equal in nature to the Father, but they are one God.
The Father is God, the Son is God, and the Holy Spirit is God. Three Persons and one God.
Salvation, the work of the Trinity
All created things have been made by God, One and Triune. God created the world, although creation is attributed to the Father; God made the Redemption, although only the second Person - the Son - became a man and died on the cross; God sanctifies us, although sanctification is attributed to the Holy Spirit. Hence, when we thank God for everything he has done for us, we have to give thanks to God the Father, to God the Son and to God the Holy Spirit.
Inhabitation of the Trinity in the soul in grace
Although it is not easy to explain, it is a truth that fills us with joy to know that the man who lives in grace (without mortal sin) is a living temple of the Most Blessed Trinity (see John 14:23). From the day of our baptism, if we do not reject God for mortal sin, it is in our soul God the Father, God the Son and God the Holy Spirit.
We have God within us to sanctify us, to help us, to be with us, because he loves us. We can speak with the Most Blessed Trinity, knowing that she listens to us and listens to our supplications. We know this through faith and, even if we do not see it or feel it, it is true. When we are in grace, we are the temple of God