For Catholics, Sunday, June 3 marked The Solemnity of The Most Holy Trinity.
Jesus sent out his Apostles to make disciples of all nations baptizing them in the name of the Father, the Son and the Holy Spirit (Matt. 28:19). The revealed truth of the Holy Trinity is that there are three persons in one God.
In order to articulate the dogma of the Trinity, the Church had to develop her own terminology with the help of certain notions of philosophical origin: "substance", "person" or "hypostasis", "relation" and so on. In doing this, she did not submit the faith to human wisdom, but gave a new and unprecedented meaning to these terms, which from then on would be used to signify an ineffable mystery, "infinitely beyond all that we can humanly understand".
The Church uses (I) the term "substance" (rendered also at times by "essence" or "nature") to designate the divine being in its unity, (II) the term "person" or "hypostasis" to designate the Father, Son and Holy Spirit in the real distinction among them, and (III) the term "relation" to designate the fact that their distinction lies in the relationship of each to the others.
Because of the oneness of nature in the Blessed Trinity, the Father is entirely in the Son and in the Holy Spirit; the Son is entirely in the Father and in the Holy Spirit; and the Holy Spirit is entirely in the Father and in the Son. Each one can only be known in relation to the other. No one of the three divine Persons is outside the other, for none precedes the other in eternity, nor surpasses the other in power, nor exceeds the other in any way. This indwelling of one divine Person in the others is called circumincession.
By the grace of Baptism "in the name of the Father and of the Son and of the Holy Spirit", we are called to share in the life of the Blessed Trinity, here on earth in the obscurity of faith, and after death in eternal light.