The persecution under Diocletian, begun in 303, had been the most severe the Church had known. It ended not with a Christian victory but with an imperial decision: in 313, meeting at Milan, Constantine and Licinius agreed that Christians — and all others — should be free to follow their religion without hindrance.
The agreement did not make Christianity the state religion; that would come later in the century. What it did was end the state's war against the Church and restore confiscated property. The faith could now be practiced in the open.
The change was vast and not without cost. A Church that had been a persecuted minority became, within decades, a favored and then established institution — gaining freedom and influence, and facing for the first time the temptations of power.