Line 1- So Did I Echo the Words of Jacob the Jew as I too was ascended to the glory of heaven and the fearful vision of the perfection of its divine plan.

Line 1:

"So did I echo those words of Jacob the Jew as I too was ascended to the glory of heaven and the fearful vision of the perfection of its divine plan."

I think the first obvious reference being made with this line is the biblical character of Jacob (later named Israel), as the paternal figure of the tribes of Israel. But in addition, "echoing his words" would be a reference to the phrase "Terribilis est Locus Iste" ("This place is terrible", or "How terrible is this place) as Jacob had recently awoke from his sleep to bear witness to what he describes as the 'House of God' and the "Gateway to Heaven'. Thus, in this context, it is also a connection to the idea of Jacob's Ladder, and the ascent to heaven.
The phrase "Terribilis est Locus Iste" also seems to point to the church of Rennes Les Chateau and the phrase which Sauniere had inscribed above the entrance.

In another context, the ascent of Jacob (upon his ladder) also seems as a reference to the Kabbalah Tree of Life, as it has in other places been described in this fashion, but also seems likely as being described the text, "the perfection of its divine plan", as the K. Tree of Life has often been described as a plan of G*d's 10 steps of emanation.

I've always had a nagging feeling about the part "So did I Echo the Words of Jacob the Jew as I too.."  having meaning about the two 'I's' in the sentence, as being possibly also reinforced by "Echo" and "too" (two).  

It brings to mind the Two-Rider Poor Knights on a Horse image, as well as an ongoing theme of duality found in many other outside sources.  But also to the image on the front cover of the book, which in one way looks like two heads connected to two bodies that meet together as a "U".