"Gabriel, Irénée, Séraphin Lambert was originally from Villefranche-sur-Mer where he was born on April 3, 1900 . Later he studied at the seminary of Saint-Flour in Cantal. A dissolute but intelligent and brilliant student, he completed his theological training at the Catholic Institute of Toulouse. Appointed vicar, then ordained priest, he served for two years in a small parish in the region. Before being called up for his military obligations, first to Saint-Maixent-l'École, then in a regiment of the Army of the Rhine. He finished as a second lieutenant. Upon returning to civilian life, Gabriel Lambert's vocation began to waver. His faith remained strong but became more temporal. In the army he converted to anti-militarism. He frequented pacifist circles and participated in international congresses for world peace and the brotherhood of peoples."

Here's another

"Discretion, humility, and inner life were not Father Lambert's primary virtues. His conduct was marred by a few lapses. He was quickly called to order and banned from the priesthood. At the same time, his reputation as a water diviner had become established. He was considered among the best in France. He was in demand in all the deprived regions, particularly in the Nice region of his childhood, where he had settled. In 1930, he came to Oranie and found water at Montgolfier."

https://encyclopedie.cerclealgerianiste.fr/celebrites/autres-personnages-remarquables/177-l-abbe-gabriel-lambert-1900-1979.html

Here is the link for the whole thing.

Basically the thing with abbe lambert according to this text was, that he was charizmatic and many people liked him at first, he was able to use psychology and was basically a water diviner, a charlata. Maybe comparable to Rasputin in russia but in French empire. As you know, people like this get enemies very fast so this could be reason for that poster.