"I'm a bit confused by this statement, how does being here now prove that I will always be and have been forever?"
As far as the intellect concerned, it doesn't! The intellect will never be capable of stepping out of its bounds and seeing this "proof". But as far as "YOU" are concerned, the proof can be seen if you look deeply enough. The intellect is still needed to an extent, but it is like a diving board... at some point you have to leave the board and enter the water. If you get too attached to the board you miss the point of why it exists in the first place! And the same is true with the intellect.
Now with that out of the way ...
Being here NOW is the only moment you ever CAN be. Infact, it's the only moment that ever IS. There is never a time when it is not NOW. Past and future are only "apparent realities" due to the filters of our minds and the nature of this dream. They are really only illusory ideas, which are being conceived of NOW. In a sense, "past" and "future" are subordinate to "now". They are "projected" from the now. "Now" is the basic, fundamental truth.
"Now" is the sole "constant", the foundation underneath the ever-moving "past" and "future". It is like the screen onto which a movie is projected. When the movie is playing, it's hard to see the screen underneath, but without that screen, the movie could not be seen. However, the movie can be removed, and the screen still remains. The screen is not dependant on the movie, but the movie is dependant upon the screen. Similarily, "Now" always *IS*, and the whole illusion of time exists "on top" of "now".
How does this "prove" (remember, it won't be much proof as far as the blind intellect is concerned) that being here now, is equivelant to being here eternally?
Because NOW *IS* ETERNITY. Now never changes. Form changes! The world changes! Thought changes! But NOW never changes. Now is eternally Now.
NOW is ETERNAL, and ETERNITY can only be found in the NOW.
And in that "now" is also the core essence of your -- our -- being. The eternally present witness. Infact, you could say that the "witness" *IS* the "now". The body changes... Bodies come and go just like clothes, but beneath form and thought the witness stays the same, just as NOW always remains NOW. This eternal witness is the source of your ability to cognize in this very moment. If this "fundamental" consciousness didn't exist, then there would be no possibility for you to be conscious and aware RIGHT NOW.
"How do you know that "eternity" is my nature?"
Because I know myself, and so I know you. Only forms differ. The essence behind both is the same. I don't know your physical form, or your mind form, or even your "soul form" (which is simply another, finer, level of manifestation) but I know the core essence, the core "reality" that is the very nature of who you REALLY are. Clay can be shaped into a bowl and a cup, but "beneath" the forms of "bowl" and "cup", the essence still remains that of clay.
"have you experienced eternity?"
Of course! This is an absurd question, but it only stems form not realizing that eternity is here right in front of your face, present in everything that you do. :)
I'm experiencing it right now, and so are you. You just don't see it because it's "covered up" by so many layers of "mind-stuff" and Maya (the "force" that causes The One Truth to appear differentiated and divided, to appear as something other than it really is).
But here's the thing ... when I say "I AM and so I will ALWAYS BE", I don't mean this bodily form or this intellect or even this expansive mind. All of those things will pass. But they are not the core essence of being. They're only clothes, or layers of manifested form (even thought is form, just less dense). The core essence of being -- of who you are, of who I am -- is inextricably linked to the NOW. They can never be seperate. And the NOW remains forever unchanged, even with the dissolution of all existence. Thus, YOU remain forever untouched. Forever pristine. Forever pure, eternal. Forever here, now, present.
But none of these things make any difference if you try to understand them simply through the intellect. These words are meant for the swimming pool, not the diving board. So meditate on it. Contemplate. Be present and aware, NOW, and know yourself!
Consider these questions:
"How do I know I exist?"
"How do I know I'm conscious?"
"How do I know I'm me?"
"Where does the past end and the now begin, and where does the now end and the future begin?" (I had fun trying to figure this one out back in my teenage days!)
Some other things:
"If we were placed here by God, then wouldn't there be purpose behind what He was doing?"
We were/are not "created" by God through any act of "doing". A stream exists as an overflowing from a source, a spring or whatever. The spring doesn't "do" anything to create the stream. The stream exists merely as a result of the spring's over-abundance.
Similarily we are a natural, inevitable "by-product" of God's "over-flowing" nature -- call it Love, or whatever you want, but it is limitless and overflowing, and hence, we are here now! He didn't / doesn't have to "do" anything. We are merely a "result" of the simple fact that God *IS*. He "exists" (for lack of a better word), and so, automatically, so do we! We are the "overflow" of God. And the flow is eternal, it never ceases. There is a never-ending "stream" of "souls" (again, lack of a better word or even mental concept, hehe) overflowing from this "God-source".
Now, one thing to realize, is that a stream is never seperate from its source. The stream is only an "extension" of the source. They are two sides of the same coin. If the source comes to an end, so does the stream. But fortunately, this isn't possible with our source. ;) Infact, only the opposite is possible. Our "source" only becomes more and more abundant, more and more expansive, overflowing more and more at an ever-increasing rate. "His" very nature is abundance and "overflowing-ness", and so too, is it ours.
"So, shouldn't we discover what our purpose is?"
You are the creator of your purpose. God did not give you any purpose. God simply loves and accepts you as you are, as a part of him (or as his "children" if you prefer to see it in that way). You are his "offspring" that arise automatically by the simple fact that he exists. He sees you arising from himself and says "OH! Did that come from me?? HAHA, THAT'S FUNNY. I LOVE YOU!!" :) He didn't "create" you with some purpose in mind. We only think of it that way because we're stuck in the intellect, and the intellect has to know a "purpose" or "reason" behind everything. Move out of the intellect and into BEING, and suddenly "purpose" loses meaning.
If you feel a need to know your purpose -- and most people do -- then find it, but know that you'll only "find" it when you realize that you're the one "creating" it.