Sometimes, the truth that we are just and sinful at the same time is misunderstood.
It is rightly recognized that, even as Christians, we still sin and still have sinful dispositions. So the “sinful” part of the picture is usually understood correctly.
But the “just” part of the equation is sometimes understood to mean to refer to what we will one day be — namely, perfectly righteous in our character and actions.
In other words, it is thought that when God looks upon as as what we are, he sees that we are still sinful and imperfect. But when he looks upon what we will one day be, he considers us just. So we are sinful in regard to what we are now, but we are just in regard to what we will be.
While it is true that we will be perfectly holy one day (and that we are died to sin now), this is not what is meant by the fact that we are at the same time just and sinful.
As I mentioned, it is true that we are sinful in respect to our character and actions right now — we are imperfect in ourselves. But we are just not because God looks out to what we will be at the judgment, but because he looks to Christ and regards Christ’s own righteousness as ours.
God considers us “just” not because of our future holiness, but because of Christ’s righteousness — which we have right now.
We are right now fully, completely, 100% just and righteous before God because the gospel means that he regards us not as we are in ourselves, but rather as we are in Christ. And the righteousness in Christ by which we are fully just before God is not the internal righteousness that he is working in our character (either now or perfected in the future), but rather an alien righteousness — a righteousness accomplished by another and reckoned as belonging to us.
So the sense in which the Christian is simultaneously just and sinful is this: we are sinful in relation to what we are in ourselves, but righteous in relation to how God regards us in Christ. This righteousness is outside of ourselves — it is not a righteousness of our character (either now or in the future), but the righteousness that Christ accomplished and has credited to us. God looks upon us as perfectly righteous because he has imputed Christ’s own righteousness to us.
This gives us perfect, unflinching security because, as Bunyan said, our righteousness is in heaven. And, as the great hymn “Before the Throne of God” says, “we know that while in heaven he stands [which is forever — times a trillion!] no tongue can bid me thence depart.”