TWO PROOFS

The phrase 'in their hearts' is a Biblical expression that refers to the mind, soul, or spirit of each person born into this world. It is where God has placed the awareness of eternity.

He has put eternity in their hearts ...  Ecclesiastes 3:11

In every human soul, a cognizance of 'something more' exists. It is the God-given attribute that separates us from all other living things ... affirmation that man is indeed created in his Father's image and for His purpose. 

We possess an innate understanding that there is much more to life than what we experience in the 'here and now'. With that knowledge comes the hope that we can one day find the fulfillment not found in this earthly corridor through which we pass ... that our salient emptiness will be forever sated once we put our faith in Jesus Christ as Lord.

That implanted consciousness of 'eternity' is ultimately linked to the intended impact of 'creation' on our belief in God. 

For since the creation of the world, His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that man is without excuse. Romans 1:20

So ... by these two provisions (the truth of eternity and creation itself) mankind chooses whether to surrender his life to God or deny His very existence. 

A believer knows 'in his heart' there is more than this fleeting, vaporous life ... that his soul will live on forever and this world is not his home.