It is a reference to Genesis 49:26 in the Douay-Rheims Bible and a few other translations. On the verse, Haydock’s commentary says, in part: The desire of the everlasting hills, &c. These blessings all looked forward towards Christ, called the desire of the everlasting hills, as being longed for, as it were, by the whole creation. Mystically, the patriarchs and prophets are called the everlasting hills, by reason of the eminence of their wisdom and holiness. (source)
The longing of the whole creation for Christ is mentioned in Romans 8:19-23.
The longing of the prophets and righteous men of old for Christ is mentioned in Matthew 13:17.
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