When in 313 CE Constantine the Great announced with the emperor of the eastern part – Licinius – tolerance edict in Mediolanum, the freedom of religion in the Roman Empire. In practice, this meant that the rapidly spreading Christianity would be able to get out of hiding and start publicly proclaiming the truths of faith in Christ.
In the 90s of the 4th century CE Theodosius I forbade any other denomination than Christianity. Domination was full, but there were still many abandoned temples and shrines in the Empire dedicated to Roman and other ancient deities, who for centuries guided the fate of society.
A great example of the use of these buildings is the city of Rome. Abandoned pagan temples were empty and unused and for years no one was eager to develop them. The city was so poor that it was looking for institutions / people who could take care of the facilities. As it turned out, only the Church was rich enough that it was able to take over and develop ancient buildings. The city authorities were eager to hand over buildings to the clergy. In this way, the Pantheon was first taken over, which in 609 was transformed into the church of the Holy Mother. In the seventh century itself, among others The Julian curia in the church Sant’Adriano al Foro, and the mausoleum of Hadrian was Christianized.
As you can see, the process of taking over pagan buildings took place only about 300 years after the Milan edict was announced. This is largely due to the gradual strengthening of the Church and getting rid of aversion to everything pagan.