In December, 1097, during the siege of Antioch, Peter began to have visions, mostly of St. Andrew. Peter claimed St. Andrew took him to the Church of St. Peter, inside Antioch, and showed him where the relic of the Holy Lance could be found. St. Andrew instructed Peter to tell the Crusade leaders about this, and to give the Lance to Raymond of St. Gilles when it was found. Peter did not immediately inform Raymond or the other leaders, and was visited four more times before June of 1098. He began to lose his sight in February of 1098, probably because of the famine afflicting the Crusaders, although he believed St. Andrew was punishing him.
After the Crusaders captured Antioch, Peter and Raymond began excavating the floor of the church. On June 14, 1098, Peter apparently discovered the Lance, and claimed to have been visited once more by St. Andrew that night, who told him to establish a feast day in honour of the discovery. Many people, including the papal legate Adhemar of Le Puy, believed Peter was a charlatan, and had simply brought a piece of iron with him to "find." After Adhemar's death later in 1098, Peter said Adhemar visited him to confirm the authenticity of the Lance.
The discovery of the Lance was at first considered to be a good omen, and it boosted the morale of the Crusaders when they were themselves besieged by a Muslim army. The Lance was credited with ensuring the Crusader victory in this siege, just as St. Andrew had promised. Nevertheless, Peter's reputation was tarnished because many of the nobles still did not believe him. He later claimed Christ had visited him and instructed the Crusaders to march barefoot to Jerusalem, although this was largely ignored. Other visions, from Christ, St. Andrew, Adhemar, and others, revealed divine anger at the various sins and vices of the Crusaders.
On April 8, 1099, Peter went through an ordeal by fire in an attempt to prove himself. It is very likely that he was severely burned in the process, although he claimed he he was uninjured because Christ had appeared to him in the fire, and that he had been hurt afterwards when a crowd rushed to him. In any case, he died on April 20.