Catholics make the sign of the cross because it is a brief profession of the Christian faith, which we received from the Apostles. 'Through the Sign of the Cross we place ourselves under the protection of the Triune God.' (CCC 2157)
As often as we make the sign of the cross in the name of the Father, and of the Son, and of the Holy Spirit, we are saying that we believe in God, a Trinity of three persons, and the Redemption of the Cross.
Calling on God by name sanctifies (blesses) the things we set out to do. It obtains for us blessings and strengthens us in difficulties and temptations. St. Cyril of Jerusalem taught the sign of the sross as a strong public witness of faith. He said,
'Let us not be ashamed to profess the Crucified One; let us confidently seal our forehead with our fingers, let us make the sign of the sross on everything, on the bread we eat and over the cup we drink. Let us make this sign as we come and go, before sleeping, when we lie down and when we arise, while traveling and while resting.'
When we make the sign of the sross, we are reminding ourselves of the purpose of the cross, remembering the price that was paid. Our Lord calls it, in Matthew 24:30, 'The Sign of the Son of Man.' St. John Chrysostom said,
'When, therefore, you sign yourself, think of the purpose of the cross, and quench any anger and all other passions. Consider the price that has been paid for you.'
The sign of the cross is our signature of faith. When we sign our name on a document or email, it tells others who we are. Likewise, making the sign of the cross shows who we are as Catholics.