Suppose a friend were to give you a very expensive watch as a gift. That would be awesome, huh?
Your friend holds out the gift to you so you reach out and take it. But instead of saying thank you for the beautifulgift you reach in your pocket, pull out some money and offer it to your friend as payment. That would be an insult to your friend wouldn't it?
A gift is something that you receive for free. You don't pay for a gift. Not even a penny. If yourfriend took even that one penny then you would have paid something towards the watch and it wouldn't be a total gift anymore.
In the same way, God offers Heaven as a free gift and when we try to do ANYTHING to get it then we are trying to pay for His FREE GIFT ourselves.