They could build their armies clandestinely or run terrorist operations, when you have the resources of a nation at your disposal a wider array of possibilities becomes available as I see it.
Anything is possible, but I don't see that kind of thing happening by the time we get to where everyone 'truly believes' in God.
That has been prophesied to happen so I believe it will happen, eventually.
Jeremiah 31:34 And they shall teach no more every man his neighbour, and every man his brother, saying, Know the Lord
: for they shall all know me, from the least of them unto the greatest of them, saith the Lord: for I will forgive their iniquity, and I will remember their sin no more.
Isaiah 11:9 They shall not hurt nor destroy in all my holy mountain:
for the earth shall be full of the knowledge of the Lord, as the waters cover the sea.
“The Day is approaching when God will render the hosts of Truth victorious, and He will purge the whole earth in such wise that within the compass of His knowledge
not a single soul shall remain unless he truly believeth in God, worshippeth none other God but Him, boweth down by day and by night in His adoration, and is reckoned among such as are well assured.”
Selections From the Writings of the Báb, pp. 153-154