First of all, we don’t “have to” make the sign of the cross before and after prayer, as if our prayer would become invalid if we don’t. It is however a very ancient tradition of the Church and I can think of many very good reasons for us to continue to practise it.
1. It is apostolic
THe earliest mention of it is from Apostolic Tradition from the 2nd century, but already it was mentioned as something widely practised. This means its earliest practice could very likely be from the first century, which was the time of the apostles. Although the mentions were uses in liturgies and sacraments, it quickly became widespread practice among the faithful whenever we pray.
2. It completes other prayers
Notice that the formula for the sign of the cross is not a complete sentence, it is only a phrase. It therefore is not a complete prayer. It was given to us since the earliest times of the Church with the sole purpose of completing other prayers.
What does it complete that may be lacking in other prayers? It ensures that our prayers are always Trinitarian: that we are addressing the God of love in One and Three, that we are part of that family and that the authority and power of our prayer comes from the Triune God. (See the next two points.)
3. It tells us who we are.
THe sign of the cross is a reminder of our baptism. As creatures, we have no access to God. But as baptized persons, children of the Father, friends of the Saviour, and temples of the Holy Spirit, our prayers are not only received by God, but are delightful to God when they are made with a sincere heart.
As we well know ourselves, however, our prayers are not always made with a pure heart nor with reverence. We tend not to trust God to be God, but expect Him to answer us as a slave.
We make the sign of the cross, and draw the cross over our body, to remind us that I belong to God, and His Will is what I seek. By mentioning God’s Name, I need to be conscious that my prayer is worthy of the Name of the good God. It must not be a careless prayer based on selfish motives and full of worldly concerns.
Indeed, I cannot pray without His Holy Spirit and it is by Christ’s blood that I am saved. So help me Lord, that this prayer may be inspired by the Holy Spirit, with trust in the Lord Jesus, for the greater glory of the loving Father awaiting me.
4. It unleashes power.
As creatures, our prayer has no power. But as a member of God’s family, as a child of God, we carry the power and authority of God Himself. Many times when we pray against evil, sickness, death, confusion, unbelief, sinfulness, fears, and all the ways of the world , the fresh and the devil, we must not pray in our own name, but in the name of the Trinity. God by adopting us has given us the right to use His Name, and His Name has absolute power over evil and sin.
By the same token, when we bless, give thanks, praise, honour, proclaim and celebrate anything good, let us not do so in our own name, which has no power, but always in the name of the Trinity, for great is His benediction and nothing rejoices God more than to bless His children over and over again.