Lets look at where our dative of means form of the noun appears with a preceding preposition, "en" which literal means in or within, or inside of something, but can also be used to show instrumentality.
1Corinthians 16:13
Be on the alert, stand firm in the faith, act like men, be strong.
Here the NASB translators chose the "en" meaning as locative rather than as showing instrumentality. However the alternate translation would read ...stand firm by faith. This choice allows translation as indicated by the grammar, rather than translating as if the noun were nominative.
1Corinthians 16:13
Be on the alert, stand firm in the faith, act like men, be strong.
Here the NASB translators chose the "en" meaning as locative rather than as showing instrumentality. However the alternate translation would read ...stand firm by faith. This choice allows translation as indicated by the grammar, rather than translating as if the noun were nominative.