21 Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you." Luke 7:20-21
So though we are all sinners we have an internal connection to the divine. We can repent and carry on until we need to repent again.
The problem I see in saying we are all sinful by nature or to our core in other words is that it teaches people that they are basically bad it seems harmful to tell people they are inherently sinful.
To tell people that they haven't lead a good life but that they are good people seem to be judging them favorably and giving them hope that they can learn how to nourish and get in touch with their true nature seems like it produces good fruits as compared with telling people they are inherently bad.
Think of most of the things that are sinful, every one of them I think is a learned behavior. Children seem so pure as compared with adults and the bad habits we pick up as we grow.
We all have the kingdom of God dwelling within us. It is up to each of us to open it up and let ourselves in and bring others too.