In 313, in the Edict of Milan, Constantine declared that all people should be free to worship as they chose. In other words, he legalized all religions, Christianity included.
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Constantine.
In 324 AD when he had defeated his co-emperor Licinius.
The Edict of Milan: Constantine I Recognised christianity and allowed people to follow this previously persecuted "new" monotheistic religion.
in A.D. 313 Constantine issued order called the edict of Milan. It gave religious freedom to all people and made Christianity legal. So the answer is, Edict of Milan.
Constantine the Great, whom the Christians proclaimed Saint after the Edict of Milan at 313 AD