JOEL - If there were undeniable scientific proof that there is a God there would be no athiests. There really is no way to "prove" the existance of God to an athiest, because a belief in God requires faith, something that true athiests do not accept so easily. Athiests will always find, or try to find, logical and scientific explanations for every physical and spiritual thing that happens on this earth. And it is hard for them to understand why any God would allow some of the terrible things that happen to people on this earth. If athiests don't believe in God then they are also not going to believe in prophets or scriptures, the truthfulness of which can only be proven by spiritual means and not scientific evidence. The bottom line is all we have is a lot of circumstantial evidence and really no direct objective scientific proof for any of these things; if there was there would be no need for religion.

This quote from Howard W. Hunter might be helpful:
"There is no tangible, concrete evidence of the existence of God or the divinity of the Master in the legal sense, but not all inquiry for truth results in proof by real or demonstrative evidence. It is fallacious to argue that because there is no demonstrative evidence of the existence of God he does not in fact exist. In the absence of evidence often thought necessary by the scientific world for positive proof, our search may take is into the realm of circumstantial evidence. We could spend hours describing the wonders of the universe, of the earth, of nature, of the human body the exactness of the laws of physics, and a thousand things, all of which dictate to the conscience of a truth seeker that there is a creator and one who rules over the universe. Suppose that all things could be proven by demonstrative evidence. What then would become of the element of faith? There would be no need for faith and it would be eliminated, giving rise then to this query: If faith is the first step or principle of the gospel and is eliminated, what happens to the gospel plan? The very foundation will crumble. I submit that there is a divine reason why all things cannot be proven by concrete evidence."
(Howard W. Hunter, "Faith - The First Step", Ensign, May 1975, 37)

An athiest has to have a fundamental change of heart to be able to accept the concept of faith and be willing to believe without the proof.

President Gordon B. Hinckley once said about the doubting Thomas:
"The other disciples therefore said unto him, We have seen the Lord." But Thomas, like so many then and now, said, "Except I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe" (John 20:25).
Have you not heard others speak as Thomas spoke? "Give us," they say, "the undeniable evidence. Prove before our very eyes, and our ears, and our hands, else we will not believe." This is the language of the time in which we live. Thomas the Doubter has become the example of men in all ages who refuse to accept other than that which they can physically prove and explain - as if they could prove love, or faith, or even such physical phenomena as electricity.
But eight days later the Apostles were together again, this time Thomas with them. "Then came Jesus, the doors being shut, and stood in the midst, and said, Peace be unto you."
Singling out Thomas, he said: "Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing."
Thomas, astonished and shaken, "answered and said unto him, My Lord and my God.
"Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed" (John 20:26-29).
Can anyone who has walked beneath the stars at night, can anyone who has seen the touch of spring upon the land doubt the hand of divinity in creation? So observing the beauties of the earth, one is wont to speak as did the Psalmist:
"The heavens declare the glory of God; and the firmament sheweth his handywork."(Ps. 19:1)
(Gordon B. Hinckley, "Be Not Faithless", Ensign, Apr. 1989, 2)

For athiests all we can do is show them through our own lives and example the importance and blessings of having faith in something that cannot be proven. We must let our own light shine before them(Matt 5:16) so that they will have a desire to have that same light within them and do what is necessary to make the change in their heart.