78 Elijah went first to Beersheba, to the mysteries and sacraments of the divine and holy Law, next he is sent to the brook, to the stream of the river which makes glad the City of God. You perceive the two Testaments of the One Author; the old Scripture as a well deep and obscure, whence you can only draw with labour; it is not full, for He Who was to fill it was not yet come, Who afterwards said: “I have come not to destroy but to fulfil the Law.” And so the Saint is bidden of the Lord to pass over to the stream, for he who has drunk of the New Testament, not only is a river, but also “from his belly shall flow rivers of living water,” rivers of understanding, rivers of meditation, spiritual rivers, which, however, dried up in the times of unbelief, lest the sacrilegious and unbelieving should drink.
Source: Letters (New Advent)