Incest in Bible

I am always wondering how Bible can self justify about incest. Why we human had to start from one single couple and inevitably produced offspring by family incest? And then later on (thousands years later) God suddenly forbidden any incest relationship without any explanation? If someone say, we started from incest because God want us to be sinful from the begining. Why? Why can God make us in a better way? If he is so powerful and kind?

3 different type of christians in this country

1. Believe in Jesus Christ in great piety. They are all nice poeple. However, I can hardly call them intelligent because all they do in their life is just follow Scripture. This type of christians take most portion of whole christian population.

2. In search of power over others. They are not honest type of people and usually quite smart about manipulating others. They are small portion among christian population.

3. Believe in God and also intelligent. They usually grew up in religious environment and well educated but somehow, they never doubt anything about Bible. They are also only small portion among whole christian population.

4. Believe in God also intelligent. They grew up in religious environment but somehow, they have a nature of questioning things. So this type of people don't take Bible literally but they still hold faith as inspiration.

Wait, did I just say "3 different type of Christians..?" Well, the last one I don't really count them as Christian.

What do you think my analysis?