Many in our culture have a very narrow view of faithfulness. They would only think of faithfulness as long as it benefits them. One can be faithful in his job, but unfaithful to his wife at the same time. How can that person be faithful in one area and not the other? It is because the job gives him the money while the affair gives him a moment that will not simple last.
The Bible teaches that God is faithful to us. One way he is faithful to us is by forgiving us of our sins once we confess them (1 John 1:9). Another way is when we are tempted, He will provide a way out of it (1 Corinthians 10:13). Since God is faithful to us, His children, it is no wonder that the work of His Spirit produces faithfulness in every believer.
As Christians, we come to Christ by grace through faith alone. That faith is simply trusting in the promises of what God has done in the finished work of Christ on the cross. The work of the Spirit that produces faithfulness is being faithful to God even in the midst of hard times. Being faithful can also means keeping our word to everyone around us, not just believers.
When it comes to discipleship, Paul told Timothy, that "what you have heard from me in the presence of many witnesses entrust to faithful men, who will be able to teach others also" (2 Timothy 2:2). One of the qualities that Timothy was to look for was faithfulness which is the fruit of the Spirit. Jesus expressed faithfulness in being our High Priest:
Since therefore the children share in flesh and blood, he himself likewise partook of the same things, that through death he might destroy the one who has the power of death, that is, the devil, and deliver all those who through fear of death were subject to lifelong slavery. For surely it is not angels that he helps, but he helps the offspring of Abraham. Therefore he had to be made like his brothers in every respect, so that he might become a merciful and faithful high priest in the service of God, to make propitiation for the sins of the people. For because he himself has suffered when tempted, he is able to help those who are being tempted.
Therefore, holy brothers, you who share in a heavenly calling, consider Jesus, the apostle and high priest of our confession, who was faithful to him who appointed him, just as Moses also was faithful in all God's house. For Jesus has been counted worthy of more glory than Moses—as much more glory as the builder of a house has more honor than the house itself. (For every house is built by someone, but the builder of all things is God.) Now Moses was faithful in all God's house as a servant, to testify to the things that were to be spoken later, but Christ is faithful over God's house as a son. And we are his house, if indeed we hold fast our confidence and our boasting in our hope (Hebrews 2:14-3:6).
The work of the Spirit is to produce Christlikeness in every believer. Just as Jesus is faithful to us, so must we also be to God and each other even when times are hard.