When a billionaire discovers the Garden of Eden, two Biblical revisionists set out to debunk him, until they learn it might be the fountain of youth.

if it is important there are 2 protagonists, then there's got to be a conflict between the 2, i think...

"when 2 Biblical scholars?investigate claims of a true garden of eded, their partnership and their faiths are tested."

>>> what is his goal?

This raises a meta question to my demented way of thinking. My initial answer would be that the billionaire has already achieved his primary objective goal, to find the Garden of Eden in Act 1 (or thereabouts). As a result of this event, what becomes the plot of movie?

Consider the character John Hammond, the huckster-developer of "Jurassic Park". At the start of the movie, he has already achieved his objective goal which is to bring dinosaurs back to life via genetic engineering and create a paleo-park to amuse (and profit from) the masses. The goal he has before him in Act 1 is to take care of a minor detail: he must secure an endorsement from 2 prominent paleontologists before the park can open.

And then the caca hits the fan. The movie becomes not a celebration of his success but a curse of the unintended consequences, of the literal and existential price he (and others) must pay for his hubris.

The arc of the plot of Jurassic Park adheres to the age-old dramatic principles of peripety, reversal of a character's fortune, and enantiodromia, the swing of the plot pendulum by Act 3 to the opposite of its state in Act 1. To wit, Hammond's Biggest Dream becomes his Biggest Nightmare. The dramatic gods always punish hubris.

So I would suggest that the goal of the plot of this logline would be the billionaire having to deal with unintended consequences of his initial success. How realizing his Biggest Dream, finding the Garden of Eden, becomes his Biggest Nightmare. How the god of the Bible and the gods of drama punish him for his hubris.(The hubris of meddling in and trying to exploit sacred objects for profane purposes -- the plot arc of "Raiders of the Lost Ark" by the way.)

And as it is the logline stops short of spelling out the plot that results from the realization it's the fountain of youth. What plot follows from that realization?

Agreed with the above.

Assuming it is the Billionaire what is his goal?

Also after the discovery of the garden and the realization that someone is trying to stop him what does he want to achieve?

Hope this helps.