of the Cross
The first certain evidence we have of the use of this sign is from the ecclesiastical
writer Tertullian (230 AD) who tells us that candidates for baptism are marked with a sign
of the cross on their foreheads during the course of their catechumenate (formation). This
doesn't mean that it was not made earlier, since as I will show it has very Jewish roots.
The Jews of Jesus' time (and orthodox Jews still do this) had the practice of wearing
certain Scripture texts in a little box on their arms and on their foreheads. They got the
idea from several passages in the Law, in particular Deut. 6:4-8:
"Hear, O Israel! The LORD our God is one. Therefore, you shall love the LORD, your
God, with all your heart, and with all your soul, and with all your strength. Take to
heart these words which I enjoin on you today. Drill them into your children. Speak of
them at home and abroad, whether you are busy or at rest. Bind them at your wrist as a
sign and let them be as a pendant on your forehead."
In this passage the first verse (the "Shema Israel") is tantamount to
Judaism's Creed - the Lord is one. This basic truth of one God who is Lord of Israel set
the People of the Old Covenant apart from the unbelieving "nations" or Gentiles.
Later in history God will say, "Pass through the city (through Jerusalem) and put
a mark on the foreheads of those who moan and groan over all the abominations that are
practiced within it" (Ezekiel 9:4). Here Ezekiel is speaking of the remnant of the
people of God who remain faithful, to set them apart from the unfaithful. The Hebrew word
for mark is the same as for the last letter of the alphabet, Tav, which in hand-written
form is cross-shaped. It was used in the ancient world to brand animals. In Greek this is
Tau, T, one of the possible shapes of Jesus' actual cross.
In the book of Revelation the servants of the Antichrist are to have their own peculiar
mark (Rev. 14:9) to indicate their belonging to him.
So, as Jews wore their sign of belief in God, as the supporters of the Antichrist will
wear his mark, it is fitting that the followers of Christ wear His mark, as the true
remnant of the people of God. This mark is preeminently the baptismal conformation to
Christ, but the Sign of the Cross is a fitting reminder since it expresses belief in the
principle Christian truths, the Trinity, Christ and the Cross which won our salvation. As
St. Paul says in 1 Cor. 1:23-24, "we proclaim Christ crucified, a stumbling block to
Jews and foolishness to Gentiles,  but to those who are called, Jews and Greeks alike,
Christ the power of God and the wisdom of God."
As for the practice of making the Sign of the Cross when passing a Catholic Church,
this is in lieu of a genuflection which one would make if one entered the church into the
Real Presence. Since Protestant churches do not have the Real Presence it would be
inappropriate to sign oneself when passing them.
Answered by Colin B. Donovan, STL