The first Missionary Oblates arrived in the Lower Rio Grande Valley of Texas in 1849. The Oblates had only recently been founded (1816) in France, for the purpose of reviving the faith among the people less touched by the Church (rural countryside, urban poor, youth, prisoners). Only very recently, in 1840, the first Oblate missionaries had begun to be sent outside of France to engage in foreign missionary work.

The Oblates in the Lower Rio Grande Valley worked out of missionary centers in Brownsville and Roma (100 miles in a direct line upriver) to take care of the whole strongly Mexican-origin South Texas area below Laredo and Corpus Christi. Their journeys on horseback to the many scattered ranches and settlements along the river and in the remote interior earned for them the title of "The Cavalry of Christ."

This image and title became immortalized through a famous photograph of some of the last horseback missionaries gathered for the church dedication in Mission in 1911. These missionaries also served the people across the border in Mexico at various places from 1855 to 1884. Several of their number died in epidemics or shipwrecks on the Gulf. One, Fr. "Pedrito" Keralum, became famous as the "Lost Missionary" when he disappeared on a visit to the ranches in the brush country in 1872, with his remains only found a decade later by some Mexican cowboys. They had to conduct their ministry in the midst of civil wars and other disturbances on both sides of the Rio Grande.

The Texas missions knew trials of every kind: revolutionary unrest, frontier bandits, inclement weather, house-wrecking hurricanes, etc. But the most terrible of all was the yellow fever which claimed many Oblate lives. On November 26, 1858, reflecting on the men who were killed in Texas, the Founder wrote: "Oh cruel mission of Texas, what frightful wounds you inflict upon my soul! This is now the fifth victim you have devoured! And I wonder how the sixth is faring under your fierce blows!"