In the Second century Tertullian said “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 occupieth us, we mark our foreheads with the sign of the cross”. So crossing ourselfs was obvious going on for some time before. You can read more here if you like: newadvent.org/cathen/13785a.htm
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