2 Thessalonians 3:3

The sentence turns on a word in the previous verse, and the wordplay only works if you read them together.

Paul has just asked for prayer that he be delivered from unreasonable and evil men — for not all have faith. Then, immediately: but the Lord is faithful. Same root, opposite subject. People are unreliable; the Lord is not. He is answering a real problem about human beings with a statement about God's character rather than a reassurance about people.

That is honest in a way that reassurance would not be. Paul does not tell the Thessalonians that the men troubling him are less dangerous than they look. He concedes the point and relocates the security.

Two verbs follow, and they work in different directions. Establish is a building word — to make firm, to set something so it holds. That works from the inside. Guard is a military word, describing a perimeter. That works from the outside.

So the promise covers both failure modes: collapsing under your own weight, and being overrun from outside. Paul knows which one his readers were facing, and does not choose between them.

And from the evil one is more likely personal than abstract in the Greek. The perimeter is being kept against somebody rather than against something.