Is it known when and why Christians began to sign themselves and impart blessings with the Cross? Among the Christians who make the sign of the cross [cf. this @fredsbend's answer], is it known when and why Christians began to sign themselves with the Cross?

The best answer will also include when imparting a blessing by tracing the sign of the cross also began and why.



My thinking: Blessing and crossing oneself are two different things. Example: say in an audience with the Pope there are many journalists attending of varied beliefs or of no belief at all. When the Pope imparts his blessing by tracing the sign of the cross, only those who cross themselves and who want to cross themselves will cross themselves. Meaning you can have one and not necessarily the other. My good guess perhaps it goes back to Christ himself or at least the Apostles.

And also there can be a blessing without tracing the sign of the cross.
The Catholic Encyclopedia says:


  The cross was originally traced by Christians with the thumb or finger
  on their own foreheads. This practice is attested by numberless
  allusions in Patristic literature.


It's unclear when it began, but it's true there are "numberless allusions" indicating it was an early and widespread tradition. For example, Tertullian:


  At every forward step and movement, at every going in and out, when we
  put on our clothes and shoes, when we bathe, when we sit at table,
  when we light the lamps, on couch, on seat, in all the ordinary
  actions of daily life, we trace upon the forehead the sign.


It's been associated with Revelation 7:


  Then I saw another angel come up from the East, holding the seal of
  the living God. He cried out in a loud voice to the four angels who
  were given power to damage the land and the sea, “Do not damage the
  land or the sea or the trees until we put the seal on the foreheads of
  the servants of our God.” I heard the number of those who had been
  marked with the seal, one hundred and forty-four thousand marked from
  every tribe of the Israelites.


Catholic commentators associate the seal on the forehead with the sign of the cross. This goes back to Ezekiel 9:4.


  Lord said to him: Pass through the city, through the midst of
  Jerusalem, and mark … the foreheads of those who grieve and lament
  over all the abominations practiced within it.


The mark spoken of here is "tau," a Hebrew letter in the shape of a cross.

The Catholic Encyclopedia also says that it wasn't long before Christians began making the sign of the cross on things other than foreheads:


  Hardly less early in date is the custom of marking a cross on objects
  — already Tertullian speaks of the Christian woman "signing" her bed
  (cum lectulum tuum signas, "Ad uxor.", ii, 5) before retiring to
  rest—and we soon hear also of the sign of the cross being traced on
  the lips (Jerome, "Epitaph. Paulæ") and on the heart (Prudentius,
  "Cathem.", vi, 129). Not unnaturally if the object were more remote,
  the cross which was directed towards it had to be made in the air.
  Thus Epiphanius tells us (Adv. Hær., xxx, 12) of a certain holy man
  Josephus, who imparted to a vessel of water the power of overthrowing
  magical incantations by "making over the vessel with his finger the
  seal of the cross" pronouncing the while a form of prayer. Again half
  a century later Sozomen, the church historian (VII, xxvi), describes
  how Bishop Donatus when attacked by a dragon "made the sign of the
  cross with his finger in the air and spat upon the monster".


As for the purpose, Athanasius quotes Antony as saying:


  When, therefore, [demons] come by night to you and wish to tell the
  future, or say, "we are the angels", give no heed, for they lie. Yea
  even if they praise your discipline and call you blessed, hear them
  not, and have no dealings with them; but rather sign yourselves and
  your houses, and pray, and you shall see them vanish. For they are
  cowards, and greatly fear the sign of the Lord's Cross, since of a
  truth in it the Savior stripped them, and made an example of them. ...
  But we by the mention of Christ crucified put all demons to flight,
  whom you fear as if they were gods. Where the sign of the Cross is,
  magic is weak and witchcraft has no strength.


Conclusion: The sign of the cross originated early, growing out of an interpretive seed found in Revelation and Ezekiel. It's a way of invoking the power of the cross against the enemies of God.