Those who know me know I think Pistis is best defined as faithfulness, which I understand as being the act of discerning what God wants one to do and the willingness to do it. So what does that mean for Hamartano?
Hamartano is understood to be "missing the mark" initially in archery, but then applied to other aspects of life, and in faith became "Sinning." So if the mark we miss is Pistis, then Sin becomes either "not discerning God in what we do" or "not acting on what we have discerned." Both of these options are things that would be hard if not impossible for anyone but the primary party to fully know.
Thus when we focus on the personal "Sin" of others, we ourselves are missing the mark of faithfulness that points throughout gospel and all stories of scripture to Justice as our primary call. Only in a just world may we even begin to be able to see others actions through shared lenses of faithfulness. As long as the world is not just, we can never clearly see the movement of God in another.
So if we are to believe God's grace is limitless, we must seek that which makes it up within ourselves: justice, love, peace, and mercy. Seeking those things of God and then acting upon them is at the core of Pistis and thus we must place them first in all we do. We may never fully accomplish this and avoid Hamartano, but when we realize this tension is inherently personal, it allows us space to more clearly see God acting in the world and seek to join in that work of bringing about God's kingdom today that is so central to our faith.