My 2 cents on the topic of the thread.

First I will make the question more precise:
So when will people that identify as atheist reach 51% of the population or more?

Possibly never.

There is a ton of confusion of what atheist, god etc even means. Perhaps if a very good completely randomized poll was created, that carefully defined the words atheist, god, and religion, in all the various languages we humans utilize. we could even begin to measure that question accurately. And the poll staff was also very careful to make the answers given were completely anonymous.
It also depends on what you (or people in general) accept as an answer? If you asked a baby that question, and predictably the baby is not able to respond to the question how is that vote counted? Atheist? Or not?

Unfortunately there really has not been any sort of world wide or even countrywide independent unbiased counting/polling of peoples religious beliefs.

It is very rare, just about anywhere in the world that there was a large scale collection of data over time of people's answer to their religious (or not religious) affiliation that would of not been heavily tainted and polluted by other variables.

For good reasons and bad, there is a lot of resistance to data collection in this manner. We are forced to look at much more subjective sources of information to try and make a guess.

We can look at signs, such as population to church attendance percentage rates. (Yeah that has dropped sharply in many areas world wide in the last century or so.) Or by examining religious influence in history compared to now. (Yeah religious influence has dropped a lot in many parts of the world as well!) But does that mean people will start identifying as "atheist" in great numbers? Who knows, I feel many people in the fast paced life of today barely give it a thought one way or another as they go about their busy days, for many people, parents and peers and possibly media told them god was real, and they accepted that, but they do not really dwell on it, consider it or plan their lives around it. They may go to church occasionally when they have the time and energy, but they do not really examine their beliefs one way or another and just go along with what they have been doing.

I think it is pretty obvious world wide religious influence is in steep, accelerating decline, but a 51% identify at atheist rate may be a long ways off, and the overall world wide trend of religious decline may reverse. A world wide calamity such as nuclear war may likely cause a large spike in religious influence, that would in turn slow and possibly reverse the percentage rate of people that identify as atheist.

 
 

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Well globally it's only Christianity, Hinduism and Islam that outnumber atheists, atheists outnumber each of the other religions. It seems odd to me that whenever numbers of atheists and secularists are compared to theists we add different theism together, why? It's not like they have anything in common really, after all monotheists disbelieve in all the deities atheists disbelieve in, except for one.

According to sociologists Ariela Keysar and Juhem Navarro-Rivera's review of numerous global studies on atheism, there are 450 to 500 million positive atheists and agnostics worldwide (7% of the world's population)

Pew research in 2015

Christian 31.2%
Muslim 24.1%
Hindus 15.1%

So the number of atheists is currently placed 4th. The more poor countries develop their economies democracy spreads, and free universal education results, the less traction religions will have to indoctrinate children into their superstitions.

I'd like to acknowledge the great contribution made by the Catholic church to global secularization.

With their pedophile priests, misogynist and homophobic ideas, and harmful rules against birth control and condom use, they are driving entire nations away from the faith. Even if they abandon all of these evil practices, as they should, the continuing discovery of past outrages will keep this momentum going.