What is the most important question that God will ask a Christian after death?
As Christians understand God, God is one and has no favourites. God is just and merciful and will judge all human beings with justice and mercy. Justice is rational and can be assessed by all human beings by the use of their reason. Every human being will encounter pure goodness and absolute truth, which is God, after death. When evil and falsehood encounter goodness and truth, this can be a painful experience: we bring judgement on ourselves and the suffering that goes with the realisation of how far short we have fallen from the high ideals set for us by God. This is the reality of judgement before God, but it is not the full picture. God’s mercy diffuses the just anger of God at our sinfulness; the mercy of God is wider than we can possibly imagine and certainly much more generous than any earthly judge.
The New Testament does not give us a catalogue of questions that must be answered at the Judgement. God is just, and therefore human beings will be judged according to what they knew to be true and right; this is an individual assessment. Only God knows what has reached the heart of each human being, and thus what they knew to be true.
The best picture-story that we have of the Day of Judgement is contained in the Gospel according to Matthew (chapter 25, verses 31 to the end). The image there is a challenge to ask if we expressed our love and devotion to God by going to the help of other human beings in need. Those who realise that this is what they are doing know that they are serving God by serving other people. Even those who do not realise this, have the potential to do what is right and go to the help of those in need; without knowing it, they are serving God and will enter into the joy of God in the life hereafter.