Romans 7:15-20
[SUP]15 [/SUP] For what I am doing, I do not understand. For what I will to do, that I do not practice; but what I hate, that I do.
[SUP]16 [/SUP] If, then, I do what I will not to do, I agree with the law that it is good.
[SUP]17 [/SUP] But now, it is no longer I who do it, but sin that dwells in me.
[SUP]18 [/SUP] For I know that in me (that is, in my flesh) nothing good dwells; for to will is present with me, but how to perform what is good I do not find.
[SUP]19 [/SUP] For the good that I will to do, I do not do; but the evil I will not to do, that I practice.
[SUP]20 [/SUP] Now if I do what I will not to do, it is no longer I who do it, but sin that dwells in me.
We sin because of our sin Nature. This is where the sin comes from even if we are walking in the Light! We have a sin nature because Adam sinned and we will not be free from this sin nature until we die.