Seems pretty straightforward! But how would we then define faithfulness? One way to put it is to say that faithfulness is simply persevering in our faith. But I think that definition is a facile one that doesn't really provide a clear enough picture of what faithfulness actually looks like. Developing a more well-rounded definition of faithfulness requires digging a little deeper! And the best way to do that is to take another look at the terminology Paul uses in defining faith, and to incorporate that terminology into our understanding of faithfulness.
Taking that approach, here is how I have come to understand faithfulness: Faithfulness is the series of tangible acts we perform in accordance with our declaration(s) of faith, until that which is not becomes that which is, ... until the substance of things hoped for becomes the reality of things attained.
Faith and faithfulness. May you and I abound in both!