Article: However, early Christian writers who speak of the shape of the particular gibbet on which Jesus died invariably describe it as having a cross-beam. For instance, the Epistle of Barnabas, which was certainly earlier than 135, and may have been of the 1st century AD, the time when the gospel accounts of the death of Jesus were written, likened it to the letter T (the Greek letter tau, which had the numeric value of 300), and to the position assumed by Moses in Exodus 17:11–12. Justin Martyr (100–165) explicitly says the cross of Christ was of two-beam shape: "That lamb which was commanded to be wholly roasted was a symbol of the suffering of the cross which Christ would undergo. For the lamb, which is roasted, is roasted and dressed up in the form of the cross. For one spit is transfixed right through from the lower parts up to the head, and one across the back, to which are attached the legs of the lamb." Irenaeus, who died around the end of the 2nd century, speaks of the cross as having "five extremities, two in length, two in breadth, and one in the middle, on which [last] the person rests who is fixed by the nails."

Question: Who says with certainty that the cross was made out of two beams?
Justin Martyr