Faith And Faithfulness "The righteous will live by faith." Hab. 2:4
The following article was written by Jacob Prasch of Moriel Ministries. I have added the bits in square brackets.
In Hebrew, the word for "faith" is "emunah" from which we get the word "amen"; in Hebrew the word for "faithfulness" is "emunah". It is the same word, like from the book of Lamentations: "Great is thy faithfulness" (Lam. 3:23). The Hebrew language makes no distinction between "faith" and "faithfulness" and neither, technically, does the Greek - "pistis". There may be a difference in the case endings, but the word for "faith" and the word for "faithfulness" is the same. In other words, if somebody's faith is genuine, they are going to be faithful. "He who believes in the Son has eternal life; but he who does not obey the Son will not see life, but the wrath of God abides on him" (John 3:36).
A saving faith is not a mere, intellectual faith; Satan has an intellectual faith; Satan knows it is true. If it is a saving faith, there will be faithfulness. The problem is that instead of following the Hebrew or Greek, our translations generally follow the Latin Vulgate of Jerome. "Faith" and "faithfulness" are different in Latin. Hence, in English, we get the wrong perspective. If it is a saving faith, we will be faithful.
Hab. 2:4 says, "The righteous will live by faith." [It could be translated,] "The righteous will live by faithfulness."
Eph. 2:8 says, "We are saved by Grace through faith." [It could be translated,] "We are saved by Grace through faithfulness." (Commencing, of course, with the faithfulness of Jesus).
Heb. 11:6 says, "Without faith it is impossible to please God." [It could be translated,] "Without faithfulness it is impossible to please God."
We make a distinction between "faith" and "faithfulness" because of translational complications we should not be making. … end of Jacob's article.
From what Jacob has shown us, we see that faith and faithfulness go hand in hand. Real faith is shown by faithfulness so if someone claims to have faith but is not faithful then they have a dead, worthless faith because James 2:26 tells us that "faith without deeds is dead." A tree is known by its fruit and "every tree that does not bear good fruit is cut down and thrown into the fire" (Matt. 7:17-19).