Why Do Catholics Make the Sign of the Cross?
The following is taken from the May/June 2003 issue of the
Oblates magazine, by the Missionary Oblates of Mary Immaculate

Initially the Sign of the Cross was made with thumb, usually
on the forehead but sometimes on the lips and chest. This small Sign of the
Cross was in common use by the end of the 4th century and is still used today
at every Mass, with the sacraments of Baptism, Confirmation, and the Anointing
of the Sick. It also is used for marking the forehead with ashes on Ash
Wednesday.
By the 6th century, people were using the first two fingers,
held together to make a large Sign of the Cross, touching the forehead, chest,
and shoulders. The two fingers symbolized the divine and human natures of
Christ.
The use of three fingers became popular in the 9th century.
The thumb and first two fingers were held outstretched together to symbolize
the Trinity, while the remaining two fingers were bent to signify Christ's two
natures. This form of the large Sign of the Cross is still used in Eastern
Christian churches, where the right shoulder is touched before the left.
By the end of the Middle Ages, the Western Church had
adopted the practice of making the large Sign of the Cross with an open hand
and touching the left shoulder before the right. This is the form we Catholics
continue to use today.
The Sign of the Cross is primarily a blessing. We use it to
call God's blessing upon us. We also use it, in both large and small versions,
to bless others or things, such as a rosary. When used in this way,the large
Sign of the Cross is made in the air. Laypersons as well as clergy can use it
to bless others. Parents,for example, may use the Sign of the Cross to bless
their children.
When we make the Sign of the Cross upon ourselves, we are
also expressing our belief in God and the Trinity. Through it, we remind
ourselves of God's love for us, of the sacrifice Jesus made to give us eternal
life, and of the presence of the Holy Spirit within us.
The Sign of the Cross also gives us a way to express our
belief in Jesus' death and our hope in the Resurrection. We embrace the cross
of Jesus and express our willingness to take up our own cross, all the while
bursting with joyful hope in the Resurrection.
The routine at Mass of making the small Sign of the Cross on
our foreheads, lips, and chest before the reading of the Gospel also has
meaning. In doing this, we acknowledge our belief in the Word of God, our
commitment to spread God's Word in our daily lives, and our awareness of God's
presence in our hearts.
So while making the Sign of the Cross may sometimes seem
like a routine action, it is not. It marks us as Christians and is a visible
expression of our belief and hope in God."
http://www.stisidore-yubacity.org/signofthecross.htm