Somewhere between 40 and 100 years after the crucifixion, and while crucifixion was still used by the Romans, Barnabas compares the shape of the cross to the Greek letter "
T" which is not the traditional shape of the Christian cross.
The Greek letter "
T" had a gematria value of
300.
English: And because the cross, which is shaped like the T, was destined to convey grace, it mentions also the "three hundred". (Holmes, 2007, p. 409)
Greek: ὅτι δὲ ὁ σταυρὸς ἐν τῷ ταῦ ἤμελλεν ἔχειν τὴν χάριν, λέγει καὶ τοὺς τριακοσίους. [9.8]
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". Wikipedia.