The profound significance of his religious profession served as a stimulus to the increase of virtues that already gave evidence of being cast in heroic mould. In demeanour he was grave and apparently without any sense of humour, yet withal possessed of a gentle and sweet disposition that greatly endeared him to those with whom he came in contact. While he could lay no claim to the great intellectual gifts and ripe scholarship that have distinguished so many of the saints of the Dominican order, he applied himself assiduously to study, and stored his mind with the sacred truths expounded in the pages of the Summa. In 1547 he was ordained to the priesthood by the Archbishop of Valencia, Saint Thomas of Villanova.
He was appointed to the most responsible office of master of novices, in the convent at Valencia, the duties of which he discharged at different intervals for an aggregate of thirty years.
The plague that decimated the inhabitants of Valencia and its vicinity in 1557 afforded the saint an excellent opportunity for the exercise of his charity and zeal. Tirelessly he ministered to the spiritual and physical needs of the afflicted. With the tenderness and devotion of a mother he nursed the sick. The dead he prepared for burial and interred with his own hands. When the plague had subsided, the zeal of the holy novice-master sought to extend the scope of his already large ministry into the apostolate of preaching. Though possessed of none of the natural qualities deemed essential for a successful career in the pulpit, he immediately attracted attention as a preacher of great force and far-reaching influence. The cathedral and most capacious churches were placed at his disposal, but proved wholly inadequate to accommodate the multitude that desired to hear him. Eventually it became necessary for him to resort to the public squares of the city. It was probably the fame of his preaching that brought him to the attention of St. Teresa, who at this time sought his counsel in the matter of reforming her order.
Unknown to his brethren, St. Louis had long cherished the desire to enter the mission fields of the New World. The hope that there he might find the coveted crown of martyrdom contributed not a little to sharpening the edge of his desire. Possessed of the necessary permission he sailed for America in 1562, and landed at Cartagena, where he immediately entered upon the career of a missionary.
From Tubera Louis went to Cipacoa and Paluato. His success at the former place, the exact location of which it is impossible to determine, was little inferior to that of Tubera. At Paluato the results of his zealous efforts were somewhat disheartening. From this unfruitful soil the saint withdrew to the province of Santa Marta, where his former successes were repeated, yielding 15,000 souls. While labouring at Santa Marta, a tribe of 1500 natives came to him from Paluato to receive baptism, which before they had rejected. The work at Santa Marta finished, the tireless missionary undertook the work of converting the warlike Caribs, probably inhabitants of the Leeward Islands. His efforts among the tribesmen seem not to have been attended with any great success.
Nevertheless, Louis used the occasion again to make manifest the protection which overshadowed his ministry. According to legend, a deadly draught was administered to him by one of the native priests. Through Divine interposition, the poison failed to accomplish its purpose.
Tenerife in the Canary Islands became the next field of the saint's apostolic labours. Unfortunately, there are no records extant to indicate what was the result of his preaching. At Mompax, thirty-seven leagues south-east of Cartagena, we are told, rather indefinitely, that many thousands were converted to the faith. Several of the West India islands, notably those of St. Vincent and St. Thomas, were visited by St. Louis in his indefatigable quest for souls.
The festival known as La Tomatina is held in Buñol, Valencia, in honor of the town's patron saints, Louis Bertrand and the Mare de Déu dels Desemparats (Mother of God of the Defenseless), a title of the Virgin Mary.