If by “sin” you mean disobeying the accepted rules of your socio-religious group, then the rules come from without, but the biological urges/desires that oppose them come from within. They come from survival instincts that are the product of millions of years of evolution. Biological evolution hasn’t kept up with cultural evolution, which is why there is conflict.
If you want to live in society, you must suppress some of your biological urges in order to avoid harming others. Staying away from things/situations that you know are “wrong” but tempt you irresistibly might help, also. If you want to "explore" some of your baser instincts, perhaps a therapist could help you do so safely.
Both outer and inner worlds may be untrustworthy at times. Use your best judgment (always subject to change as more info is acquired) about which is more reliable in any given situation.