Verses with the word stedfastness in the New Testament (2 verses):

Colossians 2:5
For though I be absent in the flesh, yet am I with you in the spirit, joying and beholding your order, and the stedfastness of your faith in Christ.

2 Peter 3:17
Ye therefore, beloved, seeing ye know these things before, beware lest ye also, being led away with the error of the wicked, fall from your own stedfastness.