Why did Constantine I place a Christian symbol on his soldiers’ shields before the Battle of Milvian Bridge?

According to historical accounts, Constantine I placed a Christian symbol on his soldiers' shields before the Battle of Milvian Bridge because he believed he had received divine instruction to do so to secure victory against his rival, Maxentius.12

The specific details of this event vary among ancient sources:

• The Dream: The Christian apologist Lactantius recorded that Constantine had a dream on the night before the battle in which he was commanded to mark the "heavenly sign of God" on his soldiers' shields. He subsequently ordered them to paint the Chi-Rho—a sacred monogram of the first two letters of the Greek word for Christ—on their equipment.12
• The Vision: Eusebius of Caesarea provided a different account in his Vita Constantini, stating that Constantine and his army saw a vision of a cross in the sky accompanied by the inscription "In this, conquer" (En toutō nika). Following this, Constantine consulted with those familiar with the Christian God, confirmed the vision's significance, and vowed to defeat Maxentius.23

Throughout his life, Constantine attributed his military successes to the support of the Christian God, and this experience is widely considered a pivotal moment in his conversion to Christianity.13