Arcana Coelestia #497
497. That 'Seth' is a second Church, less celestial than the parent Most Ancient Church, but nevertheless one of the Most Ancient Churches, is clear from what has been stated already about Seth at verse 3. The situation with Churches, as has also been stated, is that little by little, as one period of time gives way to another, they decline as regards things that are vital, chiefly for the reason dealt with already.
Genesis 5:4
4
The days of Adam after he became the father of Seth were eight hundred years, and he became the father of sons and daughters.