What incident led to Constantine christian emperor to believe in Christianity?

Answer 1

According to bishop Eusebius of Caesarea, Constantine converted to Christianity when he He felt that he needed more powerful help than his army could give him and that divine aid and protection were more important. He thought that the divination of the omens pagan gods just produced useless flattering prediction for the military commander. He therefore decided to turn to "the one Supreme [Christian] God". Then, while he was praying for god to reveal himself, he and his army had a vision of a cross of light above the sun with the inscription (in Greek) "conquer by this" attached to it. That night Jesus appeared in a dream with the same sign and ordered him "to use it as a safeguard in all engagements with his enemies." At dawn Constantine ordered the making of a representation of this sign He also decided to "worship no other God save Him who had appeared to him" and enquired who that God was." He was told that he was Christ. Constantine took this as a confirmation of his judgement and determined thenceforth to devote himself to the reading of the Inspired writings." (Life of Constantine XXVIII, XXIX, XXIX and XXXII).

Eusebius does not say where or when the event took place. However, it is clear that it was not outside Rome. He also said that Constantine prepared for war against Maxentius and "[a]ssuming therefore the Supreme God as his patron, and invoking His Christ to be his preserver and aid, and setting the victorious trophy, the salutary symbol, in front of his soldiers and bodyguard" he started his march on Italy and Rome. Therefore, he must have been in Gaul.

Lactantius, a Latin historian and a protégé of Constantine, provided a different account. He said that on the night before the Battle of the Milvian Bridge, just outside Rome, Constantine had a dream of being ordered to put "the heavenly divine sign" on the shields of his soldiers.

Eusebius and Lactantius gave different descriptions of the divine sign. Eusebius gave a "description of the Standard of the Cross, which the Romans now call the Labarum." The labarium was a vexillum; that is, a standard of the legions which Constantine used. Its long staff and the cross bar which held a square cloth towards the top symbolised a cross, a symbol suited to symbolize the crucifixion of Christ. At the top of the cross there was "the symbol of the Saviour's name." This was a Chi Rho; that is, a Chi (X) with a Rho (P) juxtaposed to it. These were the first two letters for Christ in Greek (ΧΡΙΣΤΟΣ: Christos). Lactantius described the sign as a staurogram. This was Tau-Rho; that is, a tau (Τ) superimposed on a rho (Ρ). Both were Greek letters. It was first used to abbreviate the Greek word for cross in very early Greek translations of the New Testament. It expressed the idea that the cross saves and was a symbol of salvation.

The two accounts are difficult to reconcile. However, they have been merged into a popular story that Constantine saw the Chi Rho the night of the Battle of the Milvian Bridge. It is interesting to note that both writes said that this sign was not readily understood as a symbol of as Christ. In fact there is no evidence of its use prior to Constantine. It first appeared on one of his coins in 317. He started making an extensive use of it and the labarum later, during his war against Licinius.