I think the earliest Christians used to make a sign of the cross on their forehead using their thumb.
A mark on the forehead was a sign of ownership. Slaves were often marked with their master’s name on their forehead.
*Then I looked, and lo, on Mount Zion stood the Lamb, and with him a hundred and forty-four thousand who had his name and his Father’s name written on their foreheads. (Revelation 14:1)
*They will look upon his face, and his name will be on their foreheads.(*Revelation 22:4)
-Tim-