I believe in David B. Currie’s book “Rapture” he uses the works of either Tacitus or Josephus who record that at the destruction of Jerusalem and the temple by the Romans in 70 AD, there were witnesses who claimed the image of a sword appeared in the sky over the temple. The image of a cross, a sword, or the Chi-Rho can be easily confused with each other.
Possibly, the “sign” that Constantine saw in the sky was the same that occured over the temple. The first image would be the divine judgement of God that the temple, and the Judaic sacrifices, were to be destroyed by the swords of the Romans - the second would be the divine judgement of God that the Roman Empire, or at least its false temples that led to so much persecution of Christians, were likewise to be destroyed by the sword.
Thal59