Nobody can know the grave, but it is certain everyone is going there.
Nobody can know the mind of another person, but it is certain everyone needs others.
Nobody can know themselves, but it is certain everyone is someone.
Nobody can know the will of God, but it is certain everyone is directed by him.
Nobody can know the righteousness of Christ, but it is certain everyone is naked without it.
It seems to me that certainty - absolute certainty - is found only at the boundaries of our experience. Within the limits, there are a lot of shades of grey, but when we hit a wall - a point beyond which our experience and reason cannot take us - then we find ourselves face to face with certainty.