Making the sign of the cross, or simply even having a cross itself, is an ancient Christian practice, used to punctuate prayers and in battle against evil. It is something that we Protestants have fallen out of doing but is still common among Roman Catholics, the Eastern Orthodox, and the Oriental Orthodox. Here are some reflections on the sign of the cross:
Hippolytus, The Apostolic Tradition (G. Dix & H. Chadwick trans.):
And when tempted, always reverently seal thy forehead with the sign of the Cross. For this sign of the Passion is displayed and made manifest against the devil if thou makest it in faith, not in order that thou mayest be seen of men, but by thy knowledge putting it forward as a shield. (37.1)
From Palladius, The Lausiac History, (R.T. Meyer, trans. [ACW 34]):
Another time, [Dorotheus] sent me to his cistern about the ninth hour to fill a jar with water for our refreshment. As I got there I happened to see an asp down in the well and I drew no water, but went back and told him: “We perish, Father; I saw an asp in the well.” But he smiled solemnly and looked at me, then shook his head and said: “If the devil sees fit to turn himself into a serpent or a turtle in every well, and falls in our drinking supply, shall you forever remain thirsty?” And he went out and drew water from the same well, and was the first to break his thirst by swallowing. He said: “Where the cross goes, the evil of everything loses ground.” (2.4, p. 33)
Another day when the church key was lost, [Evagrius] made the Sign of the Cross in front of the lock and, giving it a push with his hand, he opened it, calling upon Christ. (38.12, p. 114)
Evagrius Ponticus, in To Eulogius 25.27, tells of a widow whose son was possessed by a prophetic demon. She overcame it with humility, “and after she had wept bitterly, imploring Christ and making the sign of the cross, the demon quickly ran away from her child before so many lashes of the whip.” (R. Sinkewicz, trans.)
In The Conferences, 8.18, Abba Serenus tells Cassian and Germanus of how two “philosophers,” overcome by jealousy, sent demons to attack Abba Antony the Great:
These most wicked demons would not dare to go near to him, now marking his forehead or breast with the Sign of the Cross, now bowing suppliantly in true prayer. (8.18.2, my trans.)
Martin Luther, in the Small Cathechism, writes:
As soon as you get out of bed in the morning, you should bless yourself with the sign of the Holy Cross and say: “May the will of God, the Father, the Son, and the Holy Spirit be done! Amen.”
In The Mountain of Silence, Kyriacos C. Markides tells of an encounter Fr. Maximos had with a demon-possessed man. Fr. Maximos cast out the demon, holding the cross he kept underneath his robes the whole time.
It has uses other than the combat against demons and temptations, of course. It is a reminder that it the Cross of Christ that has saved from the powers of sin and death, bringing us into new life forevermore. Thus, it is fitting that it punctuate our prayers.