score:6
There are no rules in Christianity about which hand you eat with. As NigelJ writes, Christianity does not consider how you eat to be important (see Matthew 15). Such rules as there were under Judaism were abolished in Christianity (and those Jewish rules did not prohibit eating with any hand).
So, given this, allow me to explain the story you read.
While there are no rules about which hand to eat with in Christianity, there is a prohibition on eating with the left hand in Islam, and I believe in Hinduism, which are the majority religions in India. So if the character had been Hindu or Muslim he would not have eaten with his left hand. Since he did, the detective deduced he is not Hindu or Muslim, in which case it's a fair bet that he is Christian, which is the third place religion. (Statistically speaking there are other alternatives, but that doesn't make for a good detective story).
So the deduction wasn't about rules in Christianity, but about Hindu and Islamic rules.
Upvote:4
And Jesus said, Are ye also yet without understanding?
Do not ye yet understand, that whatsoever entereth in at the mouth goeth into the belly, and is cast out into the draught?
But those things which proceed out of the mouth come forth from the heart; and they defile the man.
[Matthew 15: 16-18 KJV translation.]
Jesus himself made it very clear in the above passage that how men eat (the manner of how they eat it - see the whole context of the passage in Matthew 15) does not make them clean or unclean.
What comes out of men's mouths is that which defiles them, if the heart is not clean.
Nor is there any proscription in the New Testament writings about the manner of eating, as such.
There is the forbidding of meats offered to idols and there is the forbidding of consuming blood and the forbidding of eating anything that has been strangled.
That ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication: from which if ye keep yourselves, ye shall do well. Fare ye well. Acts 15:29 KJV.
These prohibitions are all to do with the sacrifice of Jesus Christ who offered himself up to God on behalf of others. Those who spiritually (in faith) partake of that sacrifice do not compromise their faith by denying its veracity in the performance of practices which are contrary to that faith.
Eating is always a matter of sacrifice. We eat living things - both animals and vegetation. We do not eat rocks and sand and earth.
Something must die, that we may eat. And there is profound truth to be experienced every time we have a meal.
So we are instructed by the apostles that there are certain things not to be eaten. That we may understand how the sacrifice of Christ is communicated, spiritually, to humanity.
But nowhere in scripture is there any mention of what hand to eat with.