There is a discussion about whether the Cross in the early Catholic Church after Constantine was a Latin Cross, or a Chresimon (Chi-Rho symbol), here and here.
And here's my response:
And it had to have been a Latin Cross. That is, a tropaeum. It was as early as 350 CE that the Laureate Cross was displayed on sarcophagi and god knows where else. It was a Latin Cross (t) surmounted by a rising Chi-Rho symbol surrounded by a laurel wreath.
Domatilla Sarcophagus, 350 CE
The "True Cross" HAD to have been a tropaeum. Likely, a disused one, probably used to deify deceased Emperors with, stashed away with two others in a second-century underground stone reservoir built by Hadrian, which itself later became "The Chapel of the Invention of the Cross." Can't get more honest than that!
Indeed, Constantine and Helena HAD to have known about the resemblence of the "True Cross" and your typical everyday, run-of-the-mill tropaeum:
Google Books - Helena Augusta: the Mother of Constantine the Great and the Legend of her Finding of the True Cross, page 182.
Helena's dispatch of part of the Cross to Constantine has the same symbolic force. The Cross protects the Christian emperor like a phylakterion, but it also serves as a tropaeum, a representation of the heavenly alliance between the emperor and the Christian God. The tropaeum may help to defeat enemies, religious enemies like Jews and pagans, as well as the enemy on the battlefield. The Cross provides a triumphus for the emperor as well as for Christianity. The part of the Cross Helena leaves behind in Jerusalem, together with the churches she builds there, transforms the city from a pagan and Jewish centre into a Christian one: a New Jerusalem. Rufinus, Expositio Symboli 12 = CC ser. lat. 20, 149: Unde sciendum est quod crux ista triumphis erat: triumphi enim insigne est tropaeum; tropaeum autem devicti hostis indicium est.* For the Cross as trophy, see R. Storch, 1970.* Ed-M: Hence we may know that the cross was that triumph: namely a sign of triumph is the trophy, and the trophy is an indication of a defeated enemy.
A Google image search will reveal an immense number of pictures and photographs of tropaea; and except for a certain monument in Romania, they are all cruciform! And even the statue on top of the monument, which one could call the tropaeum proper, displays the form of a cross.
It's on marbles, it's on statues, it's on coins. The post-Constantinian Latin Cross, like the tropaeum that preceded it, was in the shape of a modified T.