3 As in the law, therefore, and in the Gospel [likewise], the first and greatest commandment is, to love the Lord God with the whole heart, and then there follows a commandment like to it, to love one's neighbour as one's self; the author of the law and the Gospel is shown to be one and the same. For the precepts of an absolutely perfect life, since they are the same in each Testament, have pointed out [to us] the same God, who certainly has promulgated particular laws adapted for each; but the more prominent and the greatest [commandments], without which salvation cannot [be attained], He has exhorted [us to observe] the same in both.
Source: Adversus haereses (New Advent)