The Edict of Milan was a mandate authorized in 313 A.D. by the Roman emperors Constantine and Licinius declaring the practice of Christianity legal in the Roman empire. The full text of this edict can be found translated to English in the first reference link below.
The edict contains the following declarations: "no one whatsoever should be denied the opportunity to give his heart to the observance of the Christian religion, of that religion which he should think best for himself" and "it has pleased us to remove all conditions whatsoever, which were in the rescripts formerly given to you officially, concerning the Christians and now any one of these who wishes to observe Christian religion may do so freely and openly, without molestation."
In addition, the edict called for property which had been appropriated from Christians to be "restored without any hesitation or controversy at all." The decree concludes with the intention that the edict "shall be announced everywhere and brought to the knowledge of all."
The Edict of Milan began a new era of freedom from religious persecution for Christians who had faced oppression from the Roman empire since the formation of the Christianity approximately three hundred years prior. More information concerning the edict itself can be found in the third link below.
Emperor Constantine's tolerance and acceptance of Christianity paved the way for its rapid spread throughout the Roman empire and Europe. Constantine and the Edict of Milan ushered in a significant period of time in the western world in which the Christian religion and political power were extensively and elaborately interwoven. More information about Emperor Constantine and his legacy can be found in the second reference below.