Philosoraptor

Christianity teaches that a certain level of goodness is indeed instinctive for all:
"All who have sinned without the law will also perish without the law, and all who have sinned under the law will be judged by the law. For it is not the hearers of the law who are righteous before God, but the doers of the law who will be justified. When Gentiles who have not the law do by nature what the law requires, they are a law to themselves, even though they do not have the law. They show that what the law requires is written on their hearts, while their conscience also bears witness and their conflicting thoughts accuse or perhaps excuse them on that day when, according to my gospel, God judges the secrets of men by Christ Jesus." (Rom 2:12-16)

Yet we don't always pay attention to these instincts, and sometimes we even deceive ourselves about them. Thus, we receive explicit external instruction to assist us. This comes in the form of laws both civil and religious. But these laws cannot cover everything, so for true perfection we need the grace to read the law written on our hearts and the grace to carry it out - these two are external, from God - and to make the choice to cooperate with both - this is from each of us.

That's a good question. Since Christians are "born again" and given "a new heart" by God, you would indeed expect it to be instinctive.

I basically agree with xenocles' answer. It is instinctive, but Christians don't always pay attention to those instincts, and external teaching is another means of grace from God to help us.

I think that maybe you don't know as much about Christianity as you think you do. One of the central tenets is that we are all sinners, we will all sin, but that we can be forgiven and repent.