The oldest form of the sign of the cross seems to be the small cross, traced by the thumb on the forehead. It dates back to the second century. “In all our travels and movements", says Tertullian (De cor. Mil., iii),” in all our coming in and going out, in putting of our shoes, at the bath, at the table, in lighting our candles, in lying down, in sitting down, whatever employment occupies us, we mark our foreheads with the sign of the cross".
For more on this, check out the article on the sign of the cross in the Original Catholic Encyclopedia:
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