After we got the person to agree to use common sense, logic and sound rationale, we raise the question about the existence of the creator for this grand universe. Once he agrees with the existence of the creator, we prove to him that this Creator can be only one and is totally different from His creation.
1. The point of view of design. If we have an ordered universe, for sure there must be something that ordered it. This is our human experience, when we see things working according to laws, patterns, or systems, our common sense and human experience tells us that must be something that designed it and organized it.
So, it is perfectly rational to link the observation of design, the reality of order, with intelligence and power and will. This may not prove the existence of Allah but what indicates is an intelligent wise powerful being.
We take the person through stages, following common sense and reason and keep them agreeing about every issue we explain.
2. An argument based on causality. That everything that exists or happens has a cause. So, an example will be:
You are working alone in the desert and you find a mobile phone. In the desert there are many of the components that may be used to make a mobile phone; sand, minerals and others, but to say that the mobile phone came out alone is ridiculous. There has to be a designer. Certain things have to come together. We know the difference between something random and something purposely design.
We can recognize the components of design in it, even if we find something of which we do not have previous experience about.
The point here is to show that we have really good rational reasons to believe that the universe has a creator. Because the design in the universe is a pattern everywhere; we look at the alternation in the night and the day, we look at the different composition of gases, water, planets, atmosphere, etc… So, the more you look at the amazing precision that exists in the universe, just if a little degree would have changed, the universe would not exist.
And we find these evidences of design, wherever we look. And the logical rational conclusion is that there is a designer a creator. And that designer must be very powerful and very wise and intelligent.
3. Everything that begins to exist has a cause. Something must have brought it to existence. Because is against reason and common sense and human experience, that just things come spontaneous to existence. If we find right here a pink elephant, we cannot pretend that it came out of nothing.
Using common sense we must conclude that the nature of the cause of the universe must be different from the universe itself. The cause of the universe has no beginning; it never came to exists because it always has existed. As the universe is needy, this cause of the universe must be self-sufficient, as the universe is finite; its cause must be infinite. So we must conclude that this universe has a creator. That Creator is self-sufficient, intelligent and has power and is not confined by space and time. Everything else sounds crazy.
And that Creator must be one, cause if were two, one would restrict the power of the other.
When we are using common sense and reasoning, Is good always to give a few examples. Like the example of the circle, we can have a circle and make it fit inside a square, so we have a circle and a square, but we cannot both at the same time, same place. Things do not come from nothing. What came to existence must have a cause. This is basic common sense and reasonable rules.
So, believing in God is very reasonable.