As always, they offer some history about the word:
Originally, "mettle" was simply a variant spelling of the word "metal" (which dates to at least the 14th century), and it was used in all of the same senses as its metallic relative. Over time, however, "mettle" came to be used mainly in figurative senses referring to the quality of someone's character. It eventually became a distinct English word in its own right, losing its literal sense altogether. "Metal" remained a term primarily used for those hard shiny substances such as steel or iron, but it also acquired a figurative use. Today, both words can mean "vigor and strength of spirit or temperament," but only "metal" is used of metallic substances.
The aspect of the word regarding "the quality of someone's character" is our focus.
Paul exhorts the believers in Corinth:
Do you not know that those who run in a race all run, but only one receives the prize? Run in such a way that you may win. Everyone who competes in the games exercises self-control in all things. They then do it to receive a perishable wreath, but we an imperishable. (1 Corinthians 9:24-25)
Part of the definition for mettle is stamina. The "race" Paul speaks of is not a 100-meter dash... it is a marathon, an endurance race that requires an abundance of stamina. A life of faith is not a sprint from week to week. It is a marathon from the time we are born again in faith until we die. Paul is encouraging the Corinthian believers to strengthen their mettle for a long journey.
A marathon runner does not just "show up" on the day of the race and expect to compete seriously. A runner who expects to "run in such a way that they might win" prepares, practices, and trains. In a similar way, believers should prepare, practice, and train. How can we do that? We can practice G-d's ways in small, everyday matters.
G-d has said, "Do not steal" so we should not steal. That includes taking small or otherwise insignificant items from our employers: paper clips, pencils, or pens.
G-d has said, "Do not covet" so we should not covet. Any time we see something that we are tempted to covet we can avert our eyes or direct our thoughts elsewhere.
G-d has said, "Honor your father and mother". He does not provide a qualifier that says "honor your father and mother if they are worthy of honor". He just says, "honor your father and mother.” In showing our parents honor, we show ourselves worthy of honor.
All of these examples involve choices and Scripture exhorts us to choose this day whom we will serve (Joshua 24:15). It is in the regular exercise of choosing G-d's way that we build our stamina for the long race in which we run.
We want to be absolutely clear to you, our reader: this race is run for a prize but that prize is not salvation.
The prize Paul mentions is a "wreath". The Greek word is stephanos, which literally means a "crown". This wreath/crown is the reward of our efforts Messiah describes:
Blessed are you when people insult you and persecute you, and falsely say all kinds of evil against you because of Me. Rejoice and be glad, for your reward in heaven is great; for in the same way they persecuted the prophets who were before you. (Matthew 5:12)
Our reward is not salvation. If it was then our salvation would be gained or somehow maintained by our works and we know that is not the case (Ephesians 2:8-9).
Strength of Spirit
This idea of "strengthening our mettle" ties back to the first definition: vigor and strength of spirit. This vigor and strength is not a physical measure but a spiritual one. Paul informs us why we need this spiritual strength, vigor, and stamina:
For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places. Therefore, take up the full armor of God, so that you will be able to resist in the evil day, and having done everything, to stand firm. (Ephesians 6:12-13)
Paul reminds Timothy:
For God has not given us a spirit of timidity, but of power and love and discipline. (2 Timothy 1:7)
As the Spirit of G-d performs His work in our lives and conforms us to the image of His Son, Yeshua (Romans 8:29) the things we do and the life that we live will not be our own life. Our own life was governed by the desires and passions of our flesh. Instead, the new life we live by the Spirit is the life of Messiah.
I have been crucified with Christ; and it is no longer I who live, but Christ lives in me; and the life which I now live in the flesh I live by faith in the Son of God, who loved me and gave Himself up for me. (Galatians 2:20)
May we discipline ourselves and strengthen our mettle to run the race, to fight the fight, and faithfully yield to the calling of the Spirit. As Messiah commanded us "Let your light shine before men in such a way that they may see your good works, and glorify your Father who is in heaven.” (Matthew 5:16)
Father may You be glorified by the good work you are doing in each of us!