If he can't have your soul, the next best thing is your effectiveness.
In other words, if by becoming a Christian the devil no longer has rights on your eternal destiny, the only thing he has left is to try to make sure you don't make a difference to anyone else. It's a damage limitation exercise for him from now on.
And so temptations come and failures come with the intention of bringing up to the point where we become undisciplined and eventually to the point where we quit trying. We give up on church because it doesn't work for us as if we're the centre of attention rather than a participant in something bigger. We stop seeking to live a God-honouring lifestyle because we want the freedom to run our own lives. We give in to temptation because it's easier than resisting it. And so it goes on.
All these things lead us to a discipleship that brings no change to our lives. Every challenge is not a temptation to overcome or an opportunity to grow but an example of how little God appears to care for us. We become happy to be heaven-bound but uninterested in becoming more like Jesus in the meantime.
Everyday we must make the choice to submit ourselves to God. To ask him to restore our effectiveness, the effectiveness that the devil has sought to take from us, but which ultimately he is powerless to do unless we give him the foothold to do so.