I say, there is only one religion anyway, and many methods.
Since there are many methods and many goals, there remains nothing to be the same in your fantasized “one religion”.
One state to achieve (or to be more precise, not something to achieve, but many things to let go of, all things that are causes for suffering to us and others), and countless roads to get to that result.
Even within Buddhism there are at least three possible states to achieve, depending on one's motivation, namely a better rebirth within samsara, mere liberation from samsara, and full awakening.
And, while the mere motivations to achieve the former two states may be steps to the motivation to achieve the latter, the actual achievements of a better rebirth within samsara and of liberation from samsara may themselves constitute huge hindrances to full awakening.
Therefore, even within Buddhism your theory does not hold water.
Outside Buddhism, one may want to achieve a rebirth in the form or formless realms through different types of concentration and penances, or an enjoyable rebirth within the desire realm through virtuous actions, which are all huge hindrances to full awakening as proposed by Buddhism.
Also, some religions such as Judaism even reject the possibility of rebirth, and propose the achievement of happiness for this life alone, which from a Buddhist viewpoint is the sure way to the lower realms, and therefore does not even qualify as Dharma or religion.
Therefore, at least from a Buddhist viewpoint, there are many roads bringing to different states or achievements, from the hell realms to full awakening.
As a further remark, from a Buddhist viewpoint, full awakening is a matter of
both abandoning (the obstructions to liberation and enlightenment)
and obtaining (the accumulations of merit and wisdom). Therefore, the theory of “just letting go” does not hold water either, at least from a Buddhist perspective.
And this is one thing I strongly believe in: to deny the methods different than the one we practice, to deny the other roads, to diminish them, to drive those that walk other paths away from their chosen path is not in line with walking our own spiritual path, this like walking it backwards...
Here you are mixing two utterly different things, 1) the right anyone has to investigate, analyze and criticize any religious path, and 2) the violation of the religious freedom of others.
Keeping this distinction in mind, one should feel free to investigate, analyze and criticize Buddhism, Hinduism, Jainism, Christianity, Judaism, Islam, Indo-Aryan religions, pre-Columbian religions, Neopaganism, or whatever, as much as one wants, as long as one does not violate anyone else's religious freedom.
Now, among the points to be investigated, analyzed and criticized within the topic of “one world religion” is the proposed essential characteristic of the religions championing this idea if not this slogan, to wit Judaism, Christianity and Islam, the three so-called Abrahamic religions, and their intrinsic and absolute intolerance of other religions, specifically the non-Abrahamic ones.
If you think that this is not true, that such intolerance does not exist, that such Abrahamic religions just propose that their own followers follow their own paths without forcing themselves or their beliefs on others, please feel free to make your point, and surely it will be discussed in a civilized way on the basis of reasons and facts.
However, what you propose instead is to ban any rational discussion on the merits and demerits of any given religion (or pretense thereof) under the cover of empty and deceitful slogans such as “one world religion” -- curiously the very same slogan or idea behind the violations perpetrated throughout the millennia by the above referred Abrahamic religions, which you are apparently so intent on defending.
So I am always very wary of the spiritual practitioners (or so called practitioners) that put down other spiritual practices, I stay away from them.
Considering the way you are intent on banning rational and unbiased discussion, and defending Abrahamic bigotry, may I conclude that you are wary of, and want to stay away from, your very own self?