What is the significance of Catholics pointing the cross on their chest before prayer?

I could'nt find a scripture in the bible for that, so I have no idea

Catholics, Orthodox, some Anglicans all make the sign of the cross either on their chest,or forehead as a sign of respect or reverence.

This is a custom or practice done for centuries.

The sign of the cross asserts the Christian doctrine of the Holy Trinity, and it affirms the doctrine of humanity's redemption through Christ's sacrifice. Catholics start and end their prayers in the name of The Father, Son, and Holy Spirit. Catholics will also bless themselves with holy water while entering a church by dipping their finger in holy water and making the sign of the cross.

The Sign of the Cross (http://www.fisheaters.com/sign.html)