Less than 100 years really... it is only in modern times that the various branches of Christianity have tried to merge... up until the early-mid 1900's, everyone identified as their denomination (i.e. Catholic, Baptist, Evangelical, etc)... people were very divisive, and fought against the other denominations... it was only around WWII that religious people tried to unite under the flag of "Christianity"
The main reason for bringing everyone under the flag of "Christian" was to outnumber the Muslims on polls... If you put Muslims vs Catholic vs Protestant vs Jehovah's Witness, etc... Muslims win... but if it is Muslims vs All Christians united... then Christians outnumber the Muslims... so its basically a trick to show higher numbers of followers
You can also blame those "godless communists" for bringing Christians together... you know... gotta put "in sky-god we trust" on the money so everyone knows god loves our money more than that evil communist money
As for actually bringing Christians together to believe the same stuff... that'll never happen... the reason for 4,000+ Christian denominations is that they DONT believe the same stuff... they have different interpretations, and believe different things...
Seriously... the head of the Anglican church came out as agnostic! (The Archbishop of Canterbury, leader of 80,000,000 "Christians")
You can't possibly unite people who believe in god, with ones who don't... that's simply not possible... why would you even suggest such a thing?