In both private prayer and public worship, Christians have for many centuries (since the early third) "blessed themselves" with the sign of the cross. In public, you might see folks doing this at the beginning of prayers and at the Gloria ("Glory to the Father, and to the Son, and to the Holy Spirit..."). This has been part of my devotional practice for the last three years or so, and I can enumerate perhaps three reasons why.
- Worship is physical as well as intellectual and emotional (we want to think holistically, remember?) and this is a physical remembrance that I am sealed by the Holy Spirit into the action and benefits of Christ's atonement. All the benefits of Christ's passion and death have been granted me, and in my life with him, I continue to appropriate and await the full benefit of his resurrection.
- One ancient and helpful way of understanding the Trinity is the metaphor of divine dance: the divine persons indwell and encircle each other (Gk. "perichoresis") as different but united personalities. The Holy Spirit catches us up into this dance, and enables our participation in the life of this divine community. Among many other things, our prayers, and concretely this action, comprise our "steps" in the dance. Therefore, as my life is caught up in the eternal dance of the Holy Trinity, making the sign of the cross as acknowledgement of who I am and to whom I belong is one of the steps I make as we dance together.
- Should I be in a Pentecostal exorcist kind of mood, it reminds me and any nearby demons that I am marked as Christ's own forever. But in all seriousness, in addition to bearing witness to my life in Christ, it is an invocation of the Holy Spirit, the ruach Yahweh, the very creating power and breath of God who sweeps in to re-create and renew the people of God. If I may be unnuanced, it's about summoning the power of God to bear upon one's own life.