In the old Divine Office, all of us priests pray during the morning hour called “Prime” the Symbolum Athanasium or Athanasian Creed on Trinity Sunday.  (It is prayed even more frequently in ancient editions preceding the 1960 Roman Breviary.)  The Creed below is attributed to St. Athanasius of Alexandria in the 4th century, written to distinguish true Catholic-Christians (from Arians who denied the Divinity of Christ.)

This Creed on the Father, Son and Holy Spirit begins with the unusual and striking words, “Whosoever wills to be saved…”(Quicúmque vult salvus esse) “before all things it is necessary that he hold the Catholic faith.” This is followed by an astonishingly beautiful and powerful treatise on the Holy Trinity.  The English translation below is thanks to the Franciscan Friars of the Immaculate.

Athanasian Creed in English
Whosoever wills to be saved, before all things it is necessary that he hold the Catholic faith.
Which faith except every one do keep whole and undefiled, without doubt he shall perish eternally.
Now the Catholic faith is this, that we worship one God in Trinity, and Trinity in Unity.
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 Ghost.
But the Godhead of the Father, of the Son, and of the Holy Ghost is one, the Glory Equal, the Majesty Co-Eternal.
Such as the Father is, such is the Son, and such is the Holy Ghost.
The Father Uncreated, the Son Uncreated, and the Holy Ghost Uncreated.
The Father Infinite, the Son Infinite, and the Holy Ghost Infinite. The Father Eternal, the Son Eternal, and the Holy Ghost Eternal.
And yet they are not three Eternals, but one Eternal.
As also they are not three Uncreated, nor three Infinites, but One Uncreated, and One Infinite.
So likewise the Father is Almighty, the Son Almighty, and the Holy Ghost Almighty.
And yet they are not three Almighties, but One Almighty.
So the Father is God, the Son God, and the Holy Ghost God.
And yet they are not three Gods, but One God.
So the Father is Lord, the Son Lord, and the Holy Ghost Lord.
And yet they are not three Lords, but One Lord.
For, like as we are compelled by Christian truth to acknowledge every Person by Himself to be God and Lord, so are we forbidden by the Catholic Religion to say, there be three Gods or three Lords.
The Father is made of none, neither created, nor begotten.
The Son is of the Father alone: not made, nor created, but begotten.
The Holy Ghost is of the Father, and the Son: not made, nor created, nor begotten, but proceeding.
So there is One Father, not three Fathers; one Son, not three Sons; one Holy Ghost, not three Holy Ghosts.
And in this Trinity is nothing afore or after, nothing is greater or less; but the whole three Persons are Co-Eternal together, and Co-Equal.
So that in all things, as is aforesaid, the Unity in Trinity, and the Trinity in Unity is to be worshipped.
He therefore that wills to be saved, let him thus think of the Trinity.
But it is necessary to eternal salvation, that he also believe faithfully the Incarnation of our Lord Jesus Christ.
The right Faith therefore is, that we believe and confess, that our Lord Jesus Christ, the Son of God, is God and man.
God, of the Substance of the Father, Begotten before the worlds: and Man, of the substance of His mother, born in the world.
Perfect God, Perfect Man, of a reasoning soul and human flesh subsisting.
Equal to the Father as touching His Godhead, inferior to the Father as touching His Manhood.
Who, although He be God and Man, yet He is not two, but One Christ.
One, however, not by conversion of the Godhead into Flesh, but by taking of the Manhood into God.
One altogether, not by confusion of Substance, but by Unity of Person.
For as the reasoning soul and flesh is one man, so God and man is One Christ.
Who suffered for our salvation, descended into hell, rose again the third day from the dead.
He ascended into heaven, He sitteth on the right hand of the Father, God Almighty, from whence He shall come to judge the quick and the dead.
At whose coming all men shall rise again with their bodies, and shall give account for their own works.
And they that have done good shall go into life eternal, but they that have done evil into eternal fire.
This is the Catholic Faith, which except a man believe faithfully and firmly, he cannot be saved.

Symbolum Athanasium
Quicúmque vult salvus esse, * ante ómnia opus est, ut téneat cathólicam fidem:
Quam nisi quisque íntegram inviolatámque serváverit, * absque dúbio in ætérnum períbit.
Fides autem cathólica hæc est: * ut unum Deum in Trinitáte, et Trinitátem in unitáte venerémur.
Neque confundéntes persónas, * neque substántiam separántes.
Alia est enim persóna Patris, ália Fílii, * ália Spíritus Sancti:
Sed Patris, et Fílii, et Spíritus Sancti una est divínitas, * æquális glória, coætérna maiéstas.
Qualis Pater, talis Fílius, * talis Spíritus Sanctus.
Increátus Pater, increátus Fílius, * increátus Spíritus Sanctus.
Imménsus Pater, imménsus Fílius, * imménsus Spíritus Sanctus.
Ætérnus Pater, ætérnus Fílius, * ætérnus Spíritus Sanctus.
Et tamen non tres ætérni, * sed unus ætérnus.
Sicut non tres increáti, nec tres imménsi, * sed unus increátus, et unus imménsus.
Simíliter omnípotens Pater, omnípotens Fílius, * omnípotens Spíritus Sanctus.
Et tamen non tres omnipoténtes, * sed unus omnípotens.
Ita Deus Pater, Deus Fílius, * Deus Spíritus Sanctus.
Ut tamen non tres Dii, * sed unus est Deus.
Ita Dóminus Pater, Dóminus Fílius, * Dóminus Spíritus Sanctus.
Et tamen non tres Dómini, * sed unus est Dóminus.
Quia, sicut singillátim unamquámque persónam Deum ac Dóminum confitéri christiána veritáte compéllimur: * ita tres Deos aut Dóminos dícere cathólica religióne prohibémur.
Pater a nullo est factus: * nec creátus, nec génitus.
Fílius a Patre solo est: * non factus, nec creátus, sed génitus.
Spíritus Sanctus a Patre et Fílio: * non factus, nec creátus, nec génitus, sed procédens.
Unus ergo Pater, non tres Patres: unus Fílius, non tres Fílii: * unus Spíritus Sanctus, non tres Spíritus Sancti.
Et in hac Trinitáte nihil prius aut postérius, nihil maius aut minus: * sed totæ tres persónæ coætérnæ sibi sunt et coæquáles.
Ita ut per ómnia, sicut iam supra dictum est, * et únitas in Trinitáte, et Trínitas in unitáte veneránda sit.
Qui vult ergo salvus esse, * ita de Trinitáte séntiat.
Sed necessárium est ad ætérnam salútem, * ut Incarnatiónem quoque Dómini nostri Iesu Christi fidéliter credat.
Est ergo fides recta ut credámus et confiteámur, * quia Dóminus noster Iesus Christus, Dei Fílius, Deus et homo est.
Deus est ex substántia Patris ante sǽcula génitus: * et homo est ex substántia matris in sǽculo natus.
Perféctus Deus, perféctus homo: * ex ánima rationáli et humána carne subsístens.
Æquális Patri secúndum divinitátem: * minor Patre secúndum humanitátem.
Qui licet Deus sit et homo, * non duo tamen, sed unus est Christus.
Unus autem non conversióne divinitátis in carnem, * sed assumptióne humanitátis in Deum.
Unus omníno, non confusióne substántiæ, * sed unitáte persónæ.
Nam sicut ánima rationális et caro unus est homo: * ita Deus et homo unus est Christus.
Qui passus est pro salúte nostra: descéndit ad ínferos: * tértia die resurréxit a mórtuis.
Ascéndit ad cælos, sedet ad déxteram Dei Patris omnipoténtis: * inde ventúrus est iudicáre vivos et mórtuos.
Ad cuius advéntum omnes hómines resúrgere habent cum corpóribus suis; * et redditúri sunt de factis própriis ratiónem.
Et qui bona egérunt, ibunt in vitam ætérnam: * qui vero mala, in ignem ætérnum.
Hæc est fides cathólica, * quam nisi quisque fidéliter firmitérque credíderit, salvus esse non póterit.