Angelic Trisagion This devotion to the Blessed Trinity is the official prayer of the Order of the Blessed Trinity, otherwise known as the Trinitarians.  The devotion has been recited by them and their affiliates for centuries in praise of the Trinity.

In the name of the Father, and of the Son, and of the Holy Spirit. Amen. V. Lord, open my lips. R. And my mouth shall declare Thy praise. V. O God, come to my assistance. R. O Lord, make haste to help me. V. Glory be to the Father, and to the Son, and to the Holy Spirit, R. As it was in the beginning, is now and will be forever. Amen. The decade below is recited three times, once for each member of the Trinity. All: Holy God! Holy Strong One! Holy Immortal One, have mercy upon us. V. Our Father, Who art in heaven, hallowed be Thy name. Thy kingdom come. Thy  will be done, on earth as it is in heaven. Give us this day our daily bread and forgive us  our trespasses as we forgive those who trespass against us. And lead us not into  temptation, but deliver us from evil. Amen. The following part of the decade is repeated nine times V. To Thee, O Blessed Trinity, be praise, and honor, and thanksgiving, for ever and  ever! R. Holy, holy, holy Lord, God of hosts. Heaven and earth are filled with Thy glory. V. Glory be to the Father, and to the Son, and to the Holy Spirit, R. As it was in the beginning, is now and will be forever. Amen. End of Decade Antiphon God the Father unbegotten, only-begotten Son, and Holy Spirit, the Comforter; holy  and undivided Trinity, with all our hearts we acknowledge Thee: Glory to Thee forever. V. Let us bless the Father, and the Son with the Holy Spirit. R. Be praised and exalted above all things forever. Let us pray, Almighty, ever-living God, who has permitted us Thy servants, in our profession of  the true faith, to acknowledge the glory of the eternal Trinity, and in the power of that  majesty to adore the Unity, grant, that by steadfastness in this same faith, we may be  ever guarded against all adversity: through Christ our Lord. All: Amen All: Set us free, save us, vivify us, O Blessed Trinity! In nomine Patris et Filii et Spiritus Sancti. Amen. V. Domine, labia mea aperies. R. Et os meum annuntiabit laudem tuam. V. Deus in adiutorium meum intende. R. Domine, ad adiuvandum me festina. V. Gloria Patri et Filio et Spiritui Sancto, R. Sicut erat in principio, et nunc, et semper, et in saecula saeculorum. Amen. The decade below is recited three times, once for each member of the Trinity.  All: Sanctus Deus, Sanctus fortis, Sanctus immortalis, miserere nobis.  V. Pater noster, qui es in caelis, sanctificetur nomen tuum. Adveniat regnum tuum. Fiat  voluntas tua, sicut in caelo et in terra. Panem nostrum quotidianum da nobis hodie, et  dimitte nobis debita nostra sicut et nos dimittimus debitoribus nostris. Et ne nos  inducas in tentationem, sed libera nos a malo. Amen. The following part of the decade is repeated nine times. V. Tibi laus, Tibi gloria, Tibi gratiarum actio in saecula sempiterna, O Beata Trinitas. R. Sanctus, Sanctus, Sanctus Dominus Deus exercituum. Pleni sunt coeli et terra gloria tua. V. Gloria Patri et Filio et Spiritui Sancto, R. Sicut erat in principio, et nunc, et semper, et in saecula saeculorum. Amen. End of Decade Antiphon  Te Deum Patrem ingenitum, te Filium unigenitum, te Spiritum Sanctum Paraclitum,  sanctam et individuam Trinitatem, toto corde et ore confitemur, laudamus, atque  benedicimus: Tibi gloria in saecula. V. Benedicimus Patrem, et Filium cum Sancto Spiritu. R. Laudemus et superexaltemus eum in saecula.  Oremus.  Omnipotens sempiterne Deus, qui dedisti famulis tuis in confessione verae fidei,  aeternae Trinitatis gloriam agnoscere, et in potentia maiestatis adorare Unitatem:  quaesumus, ut eiusdem fidei firmitate, ab omnibus semper muniamur adversis. Per  Christum Dominum nostrum. All: Amen. All: Libera nos, salva nos, vivifica nos, O Beata Trinitas!