we prefer using symbols associated with space, like stars or suns. it gets across the feeling of divinity (space is big and unknowable, like something divine would be) without being a symbol of any actual religion.
other christian symbols to look out for:
other variations of the cross such as the byzantine cross, the slavonic cross, the maltese cross, the baptismal cross...
also please don't use crusader crosses or related symbolism even if it is a christian flag. I should hope i don't need to explain why the crusades were not a good thing
NEVER USE THE IRON CROSS i don't have to explain why
rosaries. again, I don't know why it needs to be said, but rosaries are not applicable to every religion
the sacred heart. this is a heart on fire with thorns around it, usually with a cross or something on top
the immaculate heart of mary, another on fire heart with swords surrounding or piercing it
crowns of thorns in general
certain depictions of halos (such as cruciform halos) are inherently christian. halos do appear frequently in other religions, but some originate with christianity, so you should do your research
the luther rose: a white flower with a red heart inside. usually identified as christian by a cross over the heart, but using it without the cross is still dicey
lambs and shepherds, in certain depictions
chalices, specifically if they have a cross or X of some kind on them
the jesus fish (ichthys). this one is self explanatory I think
check any interlocking triangles or three symbols in one, they could represent the trinity
the alpha and omega symbols in christian contexts represent jesus. obviously fine in most contexts, but be aware of that when making religion themed flags. if the two are intertwined, it's probably christian
the chi-rho, a P with an x through it, as well as other christograms
palm leaves (palm sunday)
trefoils and quatrefoils (commonly found on cathedrals, symbolizes the trinity and the four evangelists respectively)
the fleur-de-lis, specifically in a religious setting, represents lillies and thus christian purity and immaculate conception
this list is nonexhaustive! and I recommend you look into any symbols you use on flags for yourself, just as a general rule of thumb
