He carried on for years after that minting coins by the million declaring his relationship to Sol Invictus. So why placard one religion, and be so shy about sneaking the other onto a coin, if that's what he really wanted to do? There might or might not be a Christian reference, but sometimes a cross is just a cross!
Constantine was a bet hedger. Not a true
Christian and not a true Pagan. He was already morphing into a sort of monotheist with
Sol as chief deity so it wasn't a big step to Christianity. Regardless, whether a
cross is just a
cross or not, this IS an earlier example of a
cross on a coin and, considering that the cult of
Sol and that of
Christ were probably not too far apart in Constantine's religious views, I'd say that it is very likely the
cross is
Christian, at least sort of (but then,
Constantine was never really more than sort of
Christian himself).