". . .to succour the Christians under the Mahometan slavery. . ."

In the old calendar today is the feast of St John of Matha who with St Felix of Valois founded the Order of the Most Holy Trinity "for the ransoming of captives". At the time of the founding, this meant captives of the Mohammedans. Plus ça change. . .

Cardinal Schuster has this to say about today's feast:

It might almost be said of this saint that he has, as it were, a right of citizenship in the Roman Calendar, not only because he was for many years in attendance on Innocent III (1198-1216) as papal chaplain, but still more because he died on the Cœlian Hill in 1213 and was buried in the venerable church of San Tommaso in formis, near which church the little cell, where he is said to have lived, is still to be seen. His sacred body was carried then to Spain after the death of Innocent X (1644-1655). The church with the adjoining monastery – the only monument in Rome which recalls the memory of the saint – belongs to the Vatican Chapter. The great door, dating from the time of Innocent III, still exists with its striking mosaic of the Saviour between two slaves, the one white the other black. Round the border of the mosaic runs the legend: + Signum Ordinis Sanctæ Trinitatis Redemptionis Captivorum..

. . . .

The appellation of the Holy Trinity adopted by the religious order founded by St John of Matha as the sign of a great reawakening of Catholic devotion towards this august mystery of our faith. During the later centuries of the Middle Ages, there were built numerous abbeys, churches, and chapels dedicated to the Blessed Trinity, and Rome, too, had her Abbey SS Trinitatis Scottorum in the vicinity of the Basilica of St Lawrence in Damaso.

Moreover the title of the Blessed Trinity is very appropriate to a religious community which devotes itself to restoring to the children of God those most precious of all the gifts with which he has endowed them – viz., liberty and salvation. If there be a divinely inspired work in all the world, it is surely that by which men endeavoured to imitate the Blessed Trinity and to lend their co-operation in the redemption of souls.

Butler's life of St John can be found here.

The proper collect for his feast reads:

Deus, qui per sanctum Ioannem ordinem sanctissimæ Trinitatis ad redimendum de potestate Saracenorum captivos cælitus instituere dignatus es: præsta, quæsumus; ut, eius suffragantibus meritis, a captivitate corporis et animæ, ti adiuvante, liberemur. Per Dominum. Amen.

O God, Whom it pleased, by means of Saint John, divinely to establish the Order of the Most Holy Trinity for ransoming captives from the power of the Saracens; grant that by Thy help, and with his merits pleading for us, we may be freed from bodily and spiritual captivity. Through Our Lord. Amen.

It is interesting that the theme of captivity is maintained in the Pauline Rite on this day even though the founder of the Trinitarians is not mentioned. The new order commemorates St Jerome Emiliani who found his vocation as a prisoner of war and St Josephine Bakhita who as a girl was captured and sold into slavery and found the faith as a slave.