Why did countries begin to establish colonies and trading posts?

to create more space for overpopulated countries
to provide raw materials that would otherwise have to be imported
to begin new religious and commercial movements

At the time when the first colonies were established, the population of Earth was a lot smaller than today- so I don't think they did it because of overpopulation. 

The reason was to gain riches and specific goods, such as spices and gold - the closes of the given options is the middle one, to provide raw materials.