Grace is reasonably well described in the Bible. It is clear what it is in earthly terms: Someone's grace is what you have when someone more powerful or influential than you favours you - he thinks you are an 'ace dude':
1Sa:27:5: And David said unto Achish, If I have now found grace in thine eyes, let them give me a place in some town in the country, that I may dwell there: for why should thy servant dwell in the royal city with thee?
Ephesians 2:8 For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:
Ephesians 2:9 Not of works, lest any man should boast.
You will note that Ephesians 2:8 says that your own faith is insufficient - you have to have that faith, but it is then down to God to give you that faith and then consider if that faith is worthy. So if God, on a whim, then decides you are the sort of person He likes, then you're saved, which presumes you are OK for heaven.
And He does act on whims:
2nd Timothy 1:9 Who hath saved us, and called us with an holy calling, not according to our works, but according to his own purpose and grace, which was given us in Christ Jesus before the world began.
Here, "grace" seems to have been given to us by Jesus. However, this can also mean that He favoured us by sending Jesus. In the light of Eph:2:8, this latter interpretation would seem to be better.
Whereas in 1Sa:27:5, David knew he had Achish's grace, there can be no independent test to show you have God's. So no one can know if they have that grace and, God, being God, may change his mind at any time.
I should imagine this is part of what drives a Christian onwards to even greater deeds in order to become God's BFFL.