Faithfulness means "adhering firmly and devotedly, as to a person, cause, or idea" (The Free Dictionary by Farflex). In Scripture faithfulness and faith are closely related, often possible translations of the same word in Old and New Testament passages. So part of true faith is faithfulness. And faithfulness is an expression of true faith.
Faithfulness is God, becoming one of us humans, in Jesus. And in Jesus bringing his kingdom down to earth. Then dying on the cross and being resurrected to new life, to ultimately bring in the fullness of the kingdom in the new creation. Which is present now in his people, the Jesus community.
God has covenanted (a promise commitment) with humankind in Jesus. The human response is faith, manifested in faithfulness within this covenant ("a binding agreement" -The Free Dictionary by Farflex) of committed love from God. So this give and take, at its heart is a matter of being faithful, faithfulness.
Faithfulness for us in Jesus means following Jesus no matter what. Trusting in Jesus and in God's promises in Scripture when one is tempted to give up in despair, or to take matters into one's own hands. Faithfulness means to be true to the revelation of God in Christ, together as a community, and as individuals in our everyday worlds.
Look at Scripture to find out what faithfulness means and how it is expressed. It's not always pretty. Just look at the psalms. Hearts poured out to God with mask removed. Repentance. Faith. Acts and words of faith often in spite of circumstances.
Certainly we fall short. Good for our humility, understanding who we really are. Then we confess to God, and to another if we've offended someone. And we accept God's forgiveness and cleansing for us in Christ. And we go on. Seeking to follow the Savior with his followers, by the Spirit.
Father, You have been and are and ever will be faithful to us in your Son. Let us in him be faithful in return. By your Spirit. And by your grace. Amen.