Why do Catholics make the sign of the cross?
In the Sign of the Cross, we profess the deepest mysteries of the Christian Faith: the Trinity–Father, Son, and Holy Spirit–and the saving work of Christ on the Cross. The combination of the words and the action are a creed–a statement of belief. We mark ourselves as Christians through the Sign of the Cross.
The Catechism of the Catholic Church tells us that when Catholics are baptized “the sign of the cross, on the threshold of the celebration, marks with the imprint of Christ the one who is going to belong to him and signifies the grace of the Redemption Christ won for us by his cross” (CCC 1235).
Thereafter, each time we make the sign of the cross we remember that we belong to Christ.
The Christian begins his day, his prayers, and his activities with the sign of the cross: “In the name of the Father and of the Son and of the Holy Spirit. Amen.” The baptized person dedicates the day to the glory of God and calls on the Savior’s grace which lets him act in the Spirit as a child of the Father. The sign of the cross strengthens us in temptations and difficulties. (CCC 2157)
All Eastern Catholics and Eastern Orthodox make the sign of the cross as well, along with many high-church Anglicans and Lutherans.