The word Paul uses here (pistis) can be translated as faith or faithfulness. But in the context of Galatians, in which Paul speaks about how faith in Christ justifies us, it can probably be best seen as a trust founded on God's own faithfulness (that's from Matera's commentary.)
This kind of faithfulness leads us to trust that God will always be with us no matter what trials we are going through. This makes me think of an incident from the life of St. Thomas. As a young man he decided he wanted to join a new order called the Dominicans. They were mendicants--traveling preachers who depended on people to help them with food and other necessities.
Thomas was from a noble family in Italy. They wouldn't have minded if he had wanted to become a Benedictine. Since he had studied at the Abbey of Monte Cassino, they thought he could enter there and eventually become the Abbot. But the idea of him going around begging for food horrified them. His mother in particular adamantly opposed Thomas on this.
But Thomas had other ideas. He wanted to follow the poor Christ, without a position of power (in those days abbots sometimes had a good deal of power and influence). So when he entered the Dominicans and they sent him on a journey to Paris, his family intervened. His brothers went after him and took him against his will back to the family castle in Aquino.
There they kept him under a sort of house arrest. They thought he wouldn't last too long and would eventually give in. But he resisted all their pressure to make him change his mind. Finally, after about a year, they realized he wasn't going to follow their plans. Instead, Thomas was faithful to the plans that he believed God had for him. That was faithfulness in action.
Is there some area in my life where I can be more faithful to what God is asking of me right now?
Prayer to the Holy Spirit for the Gift of Courage (public domain):
Come, O Holy Spirit of Courage, uphold my soul in time of trouble and adversity, sustain my efforts after holiness, strengthen my weakness, give me courage in the trials of life, that I may never be overcome and separated from you, my God and greatest Good. Amen.
Below is another version of the Taize chant of the Veni Sancte Spiritus.