Why did Diocletian’s persecution fail to eliminate Christianity?

Diocletian’s persecution failed to eliminate Christianity primarily because the religion had already become deeply established and respected within the Roman Empire by the end of the 3rd century.1

By that time, Christians were well-represented across various social strata, including the upper classes, and had gained a measure of respectability that rendered the older, hostile prejudices against them largely ineffective. With an estimated population of 5 million Christians out of 60 million inhabitants, the faith had developed a strong community structure and institutional presence that proved resilient against imperial efforts to dismantle it.1

Furthermore, the public sentiment had shifted; many citizens and even some officials no longer harbored the blind hatred that had characterized earlier centuries. Rather than annihilating the faith, the extreme violence of the persecution caused the resolve of the believers to intensify, as the faith of the martyrs served to strengthen the commitment of the remaining community. Ultimately, the movement had grown too widespread and deeply rooted to be eradicated by state-sponsored force.12