From Doug Wilson:
I was talking to a woman one time... and she told me sheepishly about her first reaction to that great grace question hypothetically presented at the pearly gates -- "why should I let you into heaven?" The right answer of course is a variant of "because of the blood of Jesus Christ, plus nothing." She told me that her first instinctive reaction was, "Gee, I hope I remember to say that."
See how faith can so easily be turned into a work?
If you are going to ask and answer this question, I think this is a much better response (from De Regno Christi)
[When I'm asked 'Why should I let you into my heaven?'] I’ll bow and be silent. Then I’ll hear a voice,
“Father, he’s mine.”
Do you see? It's not your faith that saves. It's Christ.
Here's Spurgeon (read the whole magnificent devotion here):
Remember, therefore, it is not thy hold of Christ that saves thee–it is Christ; it is not thy joy in Christ that saves thee–it is Christ; it is not even faith in Christ, though that be the instrument–it is Christ’s blood and merits; therefore, look not so much to thy hand with which thou art grasping Christ, as to Christ; look not to thy hope, but to Jesus, the source of thy hope; look not to thy faith, but to Jesus, the author and finisher of thy faith. We shall never find happiness by looking at our prayers, our doings, or our feelings; it is what Jesus is, not what we are, that gives rest to the soul. If we would at once overcome Satan and have peace with God, it must be by “looking unto Jesus.”