Being firm is one of the main messages for the New Testament church due to the constant persecutions in which they were involved for their faith, for their beliefs, and this not only goes for the New Testament church, but for us, who may not suffer physical persecution, but the pleasures of this world lie in wait for us to change the doctrine of Christ for them.
Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.
2 Thessalonians 2:15 (KJV)
This verse begins by saying “Therefore”, which denotes that Paul was giving advice in the previous verses. And yes, prior to those verses, Paul was talking about the coming of Christ and he also said that we have been chosen for salvation, so that’s why he says, “brethren, stand fast”.