to make someone strong enough for what they have to do, or to be made strong that way
Every time this verb appears, someone is being made able to do something they could not manage on their own. It is not about muscle, and it is not about becoming braver by trying harder. The strength arrives from outside the person and is aimed at a task: Christ supplies it, and the one who receives it can then act. The noun underneath the verb is the ordinary Greek word for power, "dunamis", and the prefix means "in", so the picture is of power put into a person.
The word turns up in three settings:
- God or Christ supplies the strength, and the person who received it names him as the source, as in Philippians 4:13 and 1 Timothy 1:12.
- Someone is described as becoming steadily stronger, the way Saul grew more and more effective in Damascus in Acts 9:22.
- Believers are told to be strengthened, which is a command to keep drawing on strength that is given rather than produced, as in Ephesians 6:10 and 2 Timothy 2:1.
Greek had several words in this neighborhood, and the differences matter. στηρίζω is about making something stand firm and hold its ground. κραταιόω is about growing strong over time, the way a child does. ἰσχύω is about having the strength to do a thing, which is the result rather than the giving. This verb names the supply itself. Philippians 4:13 sets two of them side by side: Paul is able to face anything because of the one who keeps supplying him the strength.
Paul also uses the plain verb this one is built from, the same word without the prefix (δυναμόω), with no real change in sense, and at Ephesians 6:10 some manuscripts carry one and some the other. That overlap is a useful check on reading too much into the prefix.