Passage
So then, brethren, stand firm, and hold fast the instructions which ye have been taught, whether by word or by our letter.
2 Thessalonians 2:15 (DBY)
Nearby Context
2 Thessalonians 2:13 But we ought to give thanks to God always for you, brethren beloved of [the] Lord, that God has chosen you from [the] beginning to salvation in sanctification of [the] Spirit and belief of [the] truth:
2 Thessalonians 2:14 whereto he has called you by our glad tidings, to [the] obtaining of [the] glory of our Lord Jesus Christ.
2 Thessalonians 2:15 So then, brethren, stand firm, and hold fast the instructions which ye have been taught, whether by word or by our letter.
2 Thessalonians 2:16 But our Lord Jesus Christ himself, and our God and Father, who has loved us, and given [us] eternal consolation and good hope by grace,
2 Thessalonians 2:17 encourage your hearts, and establish you in every good work and word.
Study Lenses
The verse centers on "brethren", "stand", "firm", "hold", "fast", "instructions", "been", and "taught". It is saying that the verse draws attention to "brethren" and "stand", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 14's "whereto he has called you by our..." into verse 16's "But our Lord Jesus Christ himself and...", so "brethren" and "stand" belong inside that flow. In 2 Thessalonians context, the local focus is Christ, faith, and discipleship.
A plain takeaway is to answer the verse's own emphasis on "brethren" and "stand" with trust shaped by these words, not by a vague optimism outside the passage.