The word pistis can mean faith (belief) or faithfulness. In other words it can be either the cause or the effect. This can make things a bit confusing at times when reading the New Testament. Here, in Galatians 5, Paul is using the word to mean faithfulness. That is, he is seeing it as the effect not the cause.
The person with pistis can be relied upon. He is dependable. She keeps her vows. Faithfulness is a loyalty and integrity that allows one to be true to their word, not shrinking back, but courageously sticking with a friend.
Fake faithfulness is being loving without being truthful. Seeming like your loyal because you are always around, but you never confront or challenge your friend. So you are only using them for your own gain, to make you feel good, not to help them become a better person.
If the Spirit has given us life, Christ’s life, then he will control that life with love, joy, peace, patience, goodness, and faithfulness. Each of these is part of the larger whole. I can’t claim to love without being a faithful friend. I can’t claim to be in step with Christ while being disloyal to others. These fruit are the character traits of Christ himself.
So faithfulness is the fruit of faith. Faith in Christ’s life in us, and his love for us. God is the source of faithfulness. He is the most faithful being in the universe. He is always loyal and never breaks a promise. His mercies are new every morning, great is his faithfulness!
And of course our Savior was faithful also- from beginning to end, from the wilderness to the garden. All the way to the cross he never faltered- why? Because he was both loyal to the Father and also to us, his human brothers and sisters.
“To live is Christ” is a life of both faith and faithfulness. In fact you can’t have one without the other- Jesus didn’t. He was both full of faith and faithfulness. So if his life is now controlling my life, it would make sense that I would also, from faith, exhibit faithfulness. If faithfulness is a trustworthiness, and loyalty that even “swears to it’s own hurt” (Psalm 15), where would the power for this kind of dependability come from? It can only come from having nothing to lose. And that can only come from having a deep sense of assurance from an everlasting hope.
Do you remain faithful to God even in hard times? To others? To your church? Is your faithfulness exhibited by both love and truth? Do you faithfully say the hard things to your spiritual friends? Is your faithfulness deeply rooted in your faith in the indwelling love of Christ for you? Does it flow from your freedom in Christ?