“Glory be to the Father and to the Son and to the Holy Spirit as it was in the beginning is now and ever shall be, world without end. Amen.”
This is a short doxology we as Catholics pray when finishing a decade of the Rosary and after finishing praying a Psalm. It gives glory to the triune God: Father, Son and Holy Spirit. Saint John Paul II wrote that “the Most Holy Trinity is the central mystery of Christian faith and life” (cf. Catechism of the Catholic Church, no.234). This is because it is “the source of all the other mysteries of faith” (ibid). This is the doctrine which determines whether a group is Christian or not. The Church only renders those groups Christian which have a valid Baptism which is conferred in the name of the Most Holy Trinity, “the Father, and the Son, and the Holy Spirit”. Certain groups like Mormons, Jehovah’s Witnesses and Oneness Pentecostals do not past this test as they do not profess faith in the Most Holy Trinity. Mormons use the words “in the name of the Father and of the Son and of the Holy Spirit”, but not with the mind of the Church which is the Trinity; instead they profess these three divine Persons to be three gods in one Godhead (rf. History of the Church 6:474, D&C 121:32, Abraham 4:26, 5:2-16.20). Jehovah’s Witnesses fiercely reject the Trinity as a pagan concept and instead adopt a more Arian view of God in which the Father alone is God and the Son is a god as their translation of John 1:1 insists. Oneness Pentecostals adopt the Sabellian doctrine of modalism which professes the three divine Persons are three modes of one divine Person.
A common Hebrew God is ‘Elohim’ which is plural for ‘El’. The name Elohim is repeatedly used in Genesis 1 and it says, “And he said: Let us make man to our image and likeness” (1:26). The Jews and other Antitrinitarian groups interpret this as God talking to the angels, but angels are created beings of God, not creators, as the Psalms suggest. This seems to come from henotheism which is the belief in many gods but the worship of only one god which was not uncommon in ancient Israel as the Scriptures repeatedly suggest.
We see other hints of the Trinity in the Old Testament. The Psalms say, “Thou hast loved justice, and hated iniquity: therefore God, thy God, hath anointed thee with the oil of gladness above thy fellows” (44:8). Saint Paul uses this in reference to Jesus the Son of God the Father (rf. Hebrews 1:9). Another reveals Jesus as the Son of Man who ascends into Heaven as King of all nations.
“I beheld therefore in the vision of the night, and lo, one like the son of man came with the clouds of heaven, and he came even to the Ancient of days: and they presented him before him. And he gave him power, and glory, and a kingdom: and all peoples, tribes and tongues shall serve him: his power is an everlasting power that shall not be taken away: and his kingdom that shall not be destroyed.” (Daniel 7:13)
In addition to the Son, we see hints of the Holy Spirit. The Spirit is active in the creation of the world as “the spirit of God moved over the waters” (cf. Genesis 1:26). The Father promised His Spirit, saying, “I will pour out my spirit upon all flesh: and your sons and your daughters shall prophesy: your old men shall dream dreams, and your young men shall see visions” (Joel 2:8).
Jesus is identified as the Incarnate Word, which is God, and the only-begotten Son of God (rf. John 1:1.14.18). The Spirit is identified as saying what God said: “And the Holy Ghost also doth testify this to us” and “he said: And this is the testament which I will make unto them after those days, saith the Lord. I will give my laws in their hearts, and on their minds will I write them: […]” (Hebrews 10:15-16).
There are many Trinitarian references especially in the New Testament. Christ commands His apostles to baptize “in the name of the Father and of the Son and of the Holy Spirit” (cf. Matthew 28:19). He promises to send His Spirit from the Father (rf. John 14:26, 15:26). The Bible mentions the Father’s knowledge, the sanctification of the Holy Spirit and the pouring of the Blood of Jesus Christ (rf. 1 Peter 1:2). These are just some of many references.
The early Church taught the Holy Trinity as we see for example in the Church Fathers. “Our teacher of these things is Jesus Christ, who also was born for this purpose,” says Saint Justin Martyr, “and was crucified under Pontius Pilate, procurator of Judea, in the times of Tiberius Caesar; and that we reasonably worship Him, having learned that He is the Son of the true God Himself, and holding Him in the second place, and the prophetic Spirit in the third, we will prove” (First Apology, 13). A prayer says the following.
“[T]he ever-truthful God, hast fore-ordained, hast revealed beforehand to me, and now hast fulfilled. Wherefore also I praise Thee for all things, I bless Thee, I glorify Thee, along with the everlasting and heavenly Jesus Christ, Thy beloved Son, with whom, to Thee, and the Holy Ghost, be glory both now and to all coming ages. Amen.” (Martyrdom of Polycarp, 14)
The Council of Nicaea dogmatically defined the Trinity in 325 A.D. after the Arian heresy questioned this doctrine. The main topic was the divinity of Jesus Christ and His relationship with the Father as being consubstanial or “one in substance” and little was mentioned of the Holy Spirit. However, the Nicene Creed did mention that the Holy Spirit “with the Father and the Son is adored and glorified”. The First Nicene Council is accepted by the Catholic Church, the Eastern Orthodox Churches, the Oriental Orthodox Churches, the Assyrian Church of the East, and Protestant ecclesial communities (i.e. Lutherans, Anglicans, Presbyterians and Methodists).
The First Council of Nicaea, A.D. CCCXXV
“And there are three who give testimony in heaven, the Father, the Word, and the Holy Ghost. And these three are one.” (1 John 5:7)