The Jewish leaders were looking for a messiah (a Christ) who would lead them to political and military victory. Jesus won the true battle that faces humanity – the battle with sin (evil/satan). That battle has been there since the beginning. Take a look:
But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must master it.” – Gen 4:7
Would God have said that “you must master sin” if it was not possible?
Now look at Romans!
For sin shall not be your master, because you are not under law, but under grace. – Rom 6:14
To me that looks like an exact match. Paul is saying that Jesus has made a way for us to master sin.
It’s worth noting that when Paul talks of “sin” he is referring to a noun, not a verb. The sin we master is a “something” that has influence and control over us, not a “bad” action of ours.