Commentaries:
So stand firm, and hold fast to the teachings we passed on to you, whether by word of mouth or by letter. May our Lord Jesus Christ himself and God our Father (who has loved us and given us unending encouragement and unfailing hope by his grace), inspire you with courage and confidence in every good thing you say or do. (Phillips)
When we read I and II Thessalonians, it does not appear that these people were going through any hard or difficult persecution, yet things were happening within the church. He tells these people, "Holdon!" There must have been pressure coming from somewhere to turn these people away from the truths, the traditions, they had learned from the apostles.
That causes one to think that, even though their neighbors were not persecuting them, nonetheless something was happening. They were in danger of being persuaded to turn away from the things that they had been taught.
It appears as if the focus of this pressure to which they were subject was something mental, doctrinal, and theological. So he tells them to "hold fast." The words J.B. Phillips uses in his translation sound like the words spoken in war: "Hang on! Hold fast!" he says. "May God inspire you with courage!"
John W. Ritenbaugh
Endure as a Good Soldier