“The mystery of the Most Holy Trinity is the central mystery of Christian faith and life.” (CCC 234)
The simplest definition of the Trinity is “Three persons in one God” (CCC 253).
The Catechism states, “the term "person" ... designates the Father, Son and Holy Spirit in the real distinction among them” (CCC 252) Each divine person is distinct, but not separate and so not individuals. We also can't call them collectively people since they are united as one. Each person is God, whole and entire. Since God is personal, God always relates to us personally, and never as a means. Each doctrine (teaching) of the Trinity reveals something of God's incredible personal love for us. We should always be open to that love every moment of the day.
“The Trinity is One. We do not confess three Gods, but one God in three persons, the "consubstantial Trinity". The divine persons do not share the one divinity among themselves but each of them is God whole and entire” (CCC 253) This is the ultimate friendship. Better friends share more of themselves with each other. Each divine person shares all of themselves with each other divine person and so the Trinity is one God. There are various words we can use to designate God's unity, "The Church uses the term "substance" (rendered also at times by "essence" or "nature") to designate the divine being in its unity” (CCC 252).
God is love (1 Jn 4:8). One definition of love is self gift. Each person of the Trinity totally, unconditionally and infinitly gives himself to the other persons of the Trinity. Each human is a person and love is something we do. For each person of the Trinity, there is no beginning nor end to their self-gift. 'What' God is, is self-giving. Therefore God's nature is one of self-gift. Therefore love is not just something God does, but is what God is. God is love.
The only way God can relate to us is by loving us.
God already has everything since he is infinite. God created us so we can receive all that God is, which is the unconditional gift of himself to us. God wants to share his divine nature with us, since jsut as each person of the Trinity receives and gives self totally, so too should we receive all that God is and give all that we are to God including our sin. God enters our life sacramentally through baptism and enters and cleanses us of sin through reconciliation. We should ask God into our lives every day.
“God himself is an eternal exchange of love, Father, Son and Holy Spirit, and he has destined us to share in that exchange.” (CCC 221)
We can sum up the three doctrines of the Trinity (person, relationship and unity) as personal loving friendship. Since God IS a personal loving friendship, he wants us to experience all that he is and so each of us are called into this personal loving friendship with him and through him with everyone else. This is THE basis for all things Catholic. Just as we have explored this incredible reality of the Trinity so too will we explore the wonderful interconnectedness of Catholic teaching in all its beauty.
"The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the "hierarchy of the truths of faith". The whole history of salvation is identical with the history of the way and the means by which the one true God, Father, Son and Holy Spirit, reveals himself to men "and reconciles and unites with himself those who turn away from sin"." (CCC 234)