So there is not one single mantra in entire Vedas that even remotely say that only those who believe in 4 Vedas are following Yogic religion. Yes, there are mantras that explain and lead us to conclude that Vedic religion and content of the 4 Vedas – both refer to same thing.
The 4 Vedas contain codes of higher levels of truth. They are like detailed texts of Physics that contain very obvious concepts as well as more subtle concepts that can be mastered only after thorough practice and understanding. They are the foundation or first source of this Vedic religion. But just as a student of Class 6 claiming to know Physics merely because he mugged up the Einstein Paper on Theory of Relativity (that can be easily downloaded from google) would only be a subject of our laughter, in same vein, if someone claims to believe in 4 Vedas without actually knowing what Vedas mean would be equally laughable.
Most people claiming adherence to Vedic religion today actually fall in this category. And that is why despite their tall claims, they remain in miserable state in terms of their strength and impact on society.
But this would come ONLY as a conclusion of a thought process and not as a blind dogma to begin with.
– There may be several reasons – previous knowledge, past experiences, thinking capacity, preferences etc – that would color the thinking process of an individual and hence it would not be as obvious for everyone to accept Vedas as divine or ultimate texts, as it may be for a few of us.
– We believe that not everyone can be a scholar but everyone can be Dharmic (honest) for sure. So if one is honest to his or her best intentions by denying infallibility of Vedas, he or she is STILL Vedic. In fact they are MORE Vedic than those blind herds who follow Vedas simply because they were told so. Had such blind followers would have been born in some other location, they may well have become blind followers of some other texts.
– If believing in Vedas be necessary to be follower of Vedic religion, it would mean that those who could not get access to books of 4 Vedas due to geography or poverty etc can never be followers of Vedic religion. Thus Vedic religion becomes religion of ONLY the fortunate ones. And hence, the claim of Supreme in Vedas itself that the knowledge of Vedas is for all human beings regardless of gender or profession or birth goes wrong!
– In reality, the whole concept of 4 Vedas is that the knowledge within them is already within us in same manner as spokes are attached to center of a wheel. By exercising the powers of our mind smartly, we reveal the knowledge that already lies within us! Refer Yajurveda 34.5. So the study of 4 Vedas externally is also a way to unravel the knowledge within. One can do so by mugging up 4 external Vedas or shouting Kalmas asserting one’s loyalty towards it. Or start with the most innate trait of rejecting falsehood proactively, keep gaining knowledge and performing worthwhile actions to build the basic foundation. And then guided by the inner voice, move ahead to master even the external 4 Vedas or whatever else is deemed necessary for achieving higher echelons of truth by a smarter soul.
In today’s context, this later way is much more natural and practical for most of us.
– While many mantras of Vedas have fairly intuitive meanings that are easy to be grasped by most of us, yet all mantras do contain deeper meanings that can be unraveled only with further and further mind control. There is a mantra that states that just as a loyal wife only comes close to her husband, in same manner meaning of Vedic mantras are understood only by the deserving. So no one on earth can claim to have understood the Vedas properly. Everyone is just a preliminary student. And hence, no one is competent enough in first place to put precondition of allegiance to Vedas – the ultimate benchmarks – for someone to adopt Vedic religion. This would be as foolish as refusing someone admission in primary school because he did not admit that sin^2y + cos^2y = 1!
So yes, belief in 4 Vedas as divine or benchmarks of wisdom may be a natural conclusion for many of us. But this is NOT a precondition to be admitted to school of Yogic religion.
The precondition is only ONE – Accept truth by rejecting falsehood!