Hebrews 7:15-17
“And it is yet far more evident, if after the similitude of Melchisedec there arises another priest.” What is evident? The interval between the two priesthoods, the difference; how much superior He is “who was made not according to the law of a carnal commandment.” (Who? Melchisedec? Nay; but Christ.) “But according to the power of an endless life. For He testifies, You are a Priest for ever after the order of Melchisedec”; that is, not for a time, nor having any limit, “but according to the power of an endless life,” that is, by means of power, by means of “endless life.”
And yet this does not follow after, “who was made not according to the law of a carnal commandment”: for what would follow would be to say, “but according to that of a spiritual one.” However by “carnal,” he implied temporary. As he says also in another place, carnal ordinances imposed until the time of reformation.
“According to the power of life,” that is, because He lives by His own power.
4. He had said, that there is also a change of law, and up to this point he has shown it; henceforward he enquires into the cause, that which above all gives full assurance to men's minds, [I mean] the knowing the cause thoroughly; and it leads us more to faith when we have learned also the cause, and the principle according to which [the thing] comes to pass.
Source: Homilies on the Epistle to the Hebrews (New Advent)