Stand firm in your faith

I Cor 15:58

Therefore, my beloved brothers, be steadfast, immovable, always abounding in the work of the Lord, knowing that in the Lord your labor is not in vain.

Steadfast used three times all in 1 Cor - morally fixed; firm in purpose (mind)

1Thes 3:8 

For now we live, if you are standing fast in the Lord.

Standing fast (firm)  persevere.

1 Corinthians 10:12

Therefore let anyone who thinks that he stands take heed lest he fall.