CCC 2157
The sign of the cross symbolically reaffirms for Catholics two essential Christian doctrines: One God - The Holy Trinity — Father, Son, and Holy Spirit — and humankind's salvation through the cross of Christ.

"The message of the cross is foolishness to those who are perishing, but to us
 who are being saved, it is the power of God."
(1 Cor 1:17-18)