Is it true that "X" exists in reality only when we are aware of having experienced it, or are aware of our potential of experiencing it through our five sense organs, namely - eyes, ears, nose, tongue and skin?
I do not accept that proposition, or at least I do not accept that the definition of "reality" it implies is equivalent to common-use definitions such as the Oxford dictionary's:
The state of things as they actually exist, as opposed to an
idealistic or notional idea of them.
The "reality" defined by your proposition is individual and personal, defined separately for each person by their awareness and experience. The common interpretation and usage of the word, on the other hand, as represented by the Oxford definition, posits a single actual state of things at any given time, independent of people, and of their thoughts and ideas.
Or perhaps you meant the "we" in your proposition to be interpreted collectively, so that your "reality" encompasses everything that is part of any person's awareness or experience. Even ignoring some potential problems with that, it's still inconsistent with my notion of "reality", which supposes that a great many things exist and are real that no human ever has, will, or even can experience. I assert, in fact, that your definition is completely incompatible with Oxford's, and mine, in that the latter describes a reality that is independent of human thought and experience, but yours is completely dependent on human thought and experience.
In this regard, if no one has experienced "God", it means "God" doesn't and cannot exist; because, if "God" could have existed in reality, it would have already existed outside of our thoughts.
Yes, that -- at least the "doesn't" part -- follows directly from your definition of "reality", but
So what? If you want to debate whether God is real, then you first have to come to a reasonable agreement about the terms involved. Your "reality" is not the one to which I normally take the central idea of atheism to apply.
Moreover, to reach the conclusion, you are assuming that "no one has experienced 'God'". At best, that's an unsupported assumption. At worst, it's an assumption of the conclusion. You have no way to establish the truth of that claim, which is in fact directly contradicted by numerous purportedly true stories in religious literature and elsewhere of people physically perceiving or experiencing God. Some of those describe manifestly objective events, such as miraculous healings and unnatural effects on geographical features. Perhaps none of those stories are true, but they establish that your assertion about people not having experienced God is not even a generally accepted position.
Thus, "God" is merely an idea.
This has not been established by your argument.
Overall, with respect to the title question,
Isn't the knowledge of the non-existence of “God” objective?
, no, absolutely not. If we suppose, arguendo, that God does not exist, then how do you suppose anyone could perceive that nonexistence, and so establish it as objective truth per the definition you present? Failing to perceive something is quite a different thing from perceiving its absence, especially if you do not know what you should expect to perceive if that thing were present.
If God does exist in some objective form or fashion then we can suppose that someone, somewhere may perceive that, or may have done in the past, or may do in the future. Thus it is at least conceivable that the existence of God could be objectively established. The opposite, on the other hand, cannot ever be objectively established.