Philippians 4:13 is certainly in the top ten of misused verses in Scripture. Here is a short summary of its intented meaning. Read Philippians 4:10-20 for the whole account.
v13 therefore means that Paul could be content in every circumstance, good or bad, because Christ enabled him to be content. The bigger picture is that Christ is glorified when He supplies in Paul’s needs through the Philippians, or when he supplies in the needs of the Philippians some other way, or when he supplies the lesson of contentment instead of supplying their need.
Adapt the following testimony to your own circumstances, and learn the secret Paul had learned.
“The economy is collapsing, the univeristy faculty is changing the program, my phone got stolen, my unborn child passed away, my car got bumped, my health is failing, (v12) … but I can contentedly trust God and keep doing what is right with a godly attitude (v11), because Christ strengthens me (v13) in ways the world does not know, and Christ will be glorified in my testimony (v20) as much as in suddenly turning everything around in rich blessing (v19). I therefore do not seek a good outcome or some perfect relief, but I seek spiritual maturity (v17). And with such a focus, when someone is a blessing to me, I will recognise their kindness and be all the more thankful (vv10 & 14).”