This is from the 1838 English translation of Jerusalem by M. Samuels:
As you can see, he regrets hearing that in the Congress of America there is talk of establishing a religion (or, to be more exact, a dominant religion). What was he referring to?
According to Alexander Altmann, this may have been a reference to the "General Assessment Bill for Support of Christian denominations," which was debated in Congress in that year. According to Altmann, it contained the clause that "The Christian religion shall in all times coming be deemed and held to be the established Religion of this Commonwealth." The defeat of this bill was largely the effort of James Madison. I would just add that it appears to me that Alexander Altmann was mistaken, as this appears to be entirely an internal Virginia matter. On the one side there was Patrick Henry, supporting such a bill, and on the other James Madison, against it. Possibly this was indeed what Mendelssohn was referring to, and possibly this bill and its defeat was a seminal moment in the history of separation of church and state in this country, five years before the Bill of Rights. Altmann may be correct, however, that this was what Mendelssohn was referring to. If so, then he might have been mistaken himself, or he might have merely used the term Congress, but meant a Congress in America, rather than the Congress.