Luther saw no wrong with Protestants doing this. This invocation isn't exclusive to Papists.
The Sign of the Cross in the Reformation
At the time of the Reformation, things were no different. Conservative reformers such as the Church of England’s Archbishop Thomas Cranmer and, in Germany, Martin Luther did not abolish signing the cross. Still, they endeavored to appropriate its use to significant occasions such as Holy Baptism, Holy Communion, affirming the Creed, and the benediction at the end of Mass. Luther was especially concerned about retaining this meaningful act of worship, preserving nearly all of its liturgical and devotional uses. In the realm of catechesis, Luther took his lead from St. Cyril, Bishop of Jerusalem (AD 315 – 386), who, in his ancient catechism for children, remarked:
Let us not then be ashamed to confess the Crucified. Be the Cross our seal made with boldness by our fingers on our brow and in everything; over the bread we eat, and the cup we drink; in our comings in, and goings out; before our sleep, when we lie down and when we awake… It is the Sign of the faithful and the dread of evils; for He has triumphed over them in it, having made a show of them openly; for when they see the Cross, they are reminded of the Crucified; they are afraid of Him, Who hath bruised the heads of the dragon. Despise not the seal of baptism, because of the freeness of the Gift; but for this rather honor thy Benefactor. [5]
Luther thought no different. He purposed to see to it that all children of the Reformation were liberated to worship the Lord in this most meaningful and dignified expression of faith and devotion. Consequently, he encouraged the signing of the cross with the invocation in his directions for morning and evening prayer within the Small Catechism. There Luther writes:
"In the morning, when you get up, make the sign of the holy cross and say, ‘In the Name of the Father, the Son and the Holy Spirit. Amen.’ … In the evening when you go to bed, make the sign of the holy cross and say…." [6]