Very early on…but on the forehead and as + etc
“Be the cross our seal, made with boldness by our fingers on our brow and in every thing; over the bread we eat and the cups we drink, in our comings and in goings; before our sleep, when we lie down and when we awake; when we are travelling, and when we are at rest”. St. Cyril of Jerusalem
(in the early Church the did a + on the forehead) I still use this personally in addition to the common form now…
I quote this to show how often at this point it was used!
“Making the sign of the cross – as we will do during the blessing – means saying a visible and public “yes” to the One who died and rose for us, to God who in the humility and weakness of His love is the Almightly, stronger than all the power and intelligence of the world.”
-Pope Benedict XVI
Angelus
September 11, 2005
he also talks about it when he was Cardinal in the Spirit of the Liturgy…as a renewal of baptism…as a shield…etc…go read that chapter