Date: 24 Menachem Av 5564
From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer rules that the two parshiyos of a mezuzah must be written in order, correcting both the Taz and Shach who mistakenly assumed the Mechilta mandates kesidran for mezuzah when in fact it addresses only tefillin. He introduces the principle that kedushah must attach at the moment of writing, so V'hayah Im Shamoa written before Shema never acquires mezuzah sanctity and cannot be retroactively sanctified. The Tosfos proof cited by the Taz is resolved by the principle that Sefer Torah kedushah inherently encompasses mezuzah kedushah (bichlal maneh masayim), making that case inapplicable to ordinary mezuzah writing.
Source: Shu"t Chasam Sofer YD §282