To many people, conscience is almost all that they have by way of knowledge of God. This still, small voice which makes them feel guilty and unhappy before, during, or after wrongdoing, is God speaking to them. It is this which, to some extent at least, controls their conduct.
It is this which impels them to shoulder the irksome duty and choose the harder path.
Now no serious advocate of a real adult religion would deny the function of conscience, or deny that its voice may at least give some inkling of the moral order that lies behind the obvious world in which we live. Yet to make conscience into God is a highly dangerous thing to do. For one thing, as we shall see in a moment, conscience is by no means an infallible guide; and for another it is extremely unlikely that we shall ever be moved to worship, love, and serve a nagging inner voice that at worst spoils our pleasure and at best keeps us rather negatively on the path of virtue.
Conscience can be so easily perverted or morbidly developed in the sensitive person, and so easily ignored and silenced by the insensitive that it makes a very unsatisfactory god. For while it is probably true that every normal person has an embryo moral sense by which he can distinguish right from wrong, the development, non-development, or perversion of that sense is largely a question of upbringing, training, and propaganda.
Source: YOUR GOD IS TOO SMALL by J. B. Phillips