In 1947 Everson v Board of Education a liberal court took the words separation between church & state out of the letter and used just those words out of context. This is where the so-called separation of church & state comes from. This his been uses ever since to remove religion from everything. The court need to put those words back in the letter and look at the complete meaning
Personally, I’d prefer to refer to the plain text of the Constitution: CONGRESS shall, nothing about the States, which rolls to the 9th/10th.
His letter, after the signing, can be referenced, but ultimately conveys NO bearing upon the 1st.
All we need is the courts pulling phrases out of every pamphlet/letter/parchment of the time to justify their personal wants.