Human Terms

As I was reading over Romans this past week, I got to chapter six.  I have read this before but I never questioned the meaning until this past reading.

Romans 6:15-19 – What then? Shall we sin because we are not under law but under grace? Absolutely not! Do you not know that if you present yourselves as obedient slaves, you are slaves of the one you obey, either of sin resulting in death, or obedience resulting in righteousness? But thanks be to God that though you were slaves to sin, you obeyed from the heart that pattern of teaching you were entrusted to, and having been freed from sin, you became enslaved to righteousness. (I am speaking in human terms because of the weakness of your flesh.)

I am speaking in human terms.  What about this section of scripture is written because of the weakness of their flesh?