Luke 16:12: And if you have not been faithful in that which is another’s, who will give you that which is your own? (ESV)
Faithfulness is the consistent application of diligence in a particular direction. Faithfulness is a spiritual quality as listed among the fruit of the Spirit (Galatians 5:22-23). Because we needed the Holy Spirit to bear that fruit in us means it is a deficiency in us occasioned by the sin of Adam.
When there is deficiency, then we need supplement. The more we fellowship with the Holy Spirit, in the word and prayer, we get infuse with the quality of faithfulness (1Corinthians 13:14).
There is no fruit without seed, and the seed is Jesus, it is the righteousness of Jesus that becomes our overall righteousness as enhanced by the Holy Spirit (2Corinthians 5:21).
Jesus described himself as a seed that needs to fall to the ground and which produces many others (John 12:24). In his death many were made righteous (Romans 5:19). A component of the character of Jesus is his faithfulness. The bible says in the book of Hebrews that he was faithful as a son over his own house (Hebrews 3:5-6). He was consistently diligent in that direction.
At a risk to his own neck, he stood out for the house of God, upsetting the existing economic system, making a whip beating people involved in selling in the temple and generally making himself obnoxious to the religious establishment. All because, as he said: “My Father’s house shall be called a house of prayer, but you have made it a den of thieves (Matthew 21:12-13, John 2: 13-17).”
He is faithful. He said: I will build my church (Matthew 16:18). He is faithful to that also. He said he will never leave you nor forsake you (Hebrews 13:5). He is also faithful to that.