Most everyone should be Catholic because that is the truth.
Truth should be held up by the government, so the government should encourage the Church and protect it, and the government should of course be informed by Church teachings.
People have civil freedom to choose their religion, so the government should not be able to “impose” that on individuals by way of establishing any state church.
So I guess the answer to your question depends on what you mean by separation: If you mean as the Founding Fathers did, that the federal government shall not “establish” as a state religion any particular sect, requiring attendance and giving them money, I would say, no. But if you mean that the government should look only to secular principles on which to base laws: well, that’s downright dumb, and I’m against it.