The ultimate
There is a very important consequence of dealing with real values. If what you’re doing has real value then your life has (potentially at least) real value and then so does humanity and existence in general. That which is good fulfils the purpose of creation and that which is bad opposes it. This means that what you do in life matters; how you live matters. You can either live in such a way that your life contributes in some way to the benefit of your life, humanity, life in general, existence, or you don't.
If there were no purpose to existence then everything you do would be utterly worthless and futile. At the end of it all, whatever you did with your life, whatever you became is completely irrelevant; nothing you did was worthwhile.
You may as well have never existed.
If on the other hand there is purpose to existence then what you do and what you are is important. You can make a real difference. There needs to be a judgement of the fulfilment of this purpose, of the real value of your role in existence. That judgement must be by God - no one else is qualified.
It is worthwhile being alive.
We have now reached the topic of belief in God. Many people in the West have already got well-developed concepts of what God is - though this is changing with people in Europe, where religion itself is almost taboo as a serious subject. The arguments that have been presented so far are really concerning God as the ultimate source of all real value. Anything that is really good flows from God's compassion and mercy: in Arabic one would say God is Ar-Rahman.
Disbelief in God therefore has the primary implication of disbelief in morals and values. If you disbelieve in God in the sense of Ar-Rahman then you assert that none of your deeds can have real value.
The sin of disbelief in God then is a profoundly important one since it is profoundly tied up with sin in general. The subject of the concept of sin in general will be dealt with later, but for now let us consider the evidence which is relevant for this belief. The sin of disbelief in a particular revelation will also be considered later.
The strength of the argument so far lies in appealing to what makes sense to human beings. Human beings need to value and be valued and it makes sense to us that those values are real.. There is nothing weak in making such an appeal; indeed the appeal to good thinking is similarly an appeal to part of human nature. An appeal for good thinking includes an appeal for consistency with what is already accepted and for the acceptance of clear evidence. Muslims commonly know Islam as the natural way of life (din ul-fitra) because it fits this human nature perfectly. Another aspect of this argument flows from the in-built sense of justice in human nature, which requires that there are right and wrong deeds and that we are accountable for them, though to discuss this now in depth would take us off the track.
The nature of evidence which convinces us of the existence of real value is the type of evidence which brings about in us a sense of awe; of appreciation; a recognition of beauty and the conviction that all this cannot be here for no purpose; it cannot be just an accident of no consequence; it cannot ultimately be worthless like an idle game.