Philippians 4:13 is not a promise of personal achievement or magical success in every endeavor, but an assurance of divine sufficiency for whatever state of life God ordains. Paul is explaining that he has learned the secret of contentment in both prosperity and hardship because Christ is the source of his strength.
In Philippians 4:11–13, Paul speaks not of his own power to accomplish worldly ambitions, but of his capacity to endure whatever circumstances the Lord places him in. The context is vital: Paul had just described his ability to be content when 'abased' or 'hungry' just as much as when he was 'filled' or 'abounding.' His 'can do' is not a claim to superhuman ability, but a confession of his dependence upon the Lord Jesus. As John Chrysostom noted in his homilies on Romans, our confidence in the future is grounded in the fact that God has already provided for us in the past through Christ, who strengthens us to remain faithful even in distress. This is not about us becoming great, but about Christ being sufficient when we are weak.
This doctrine of divine strength is echoed in Paul’s own experience with his 'thorn in the flesh.' In 2 Corinthians 12:9–10, the Lord tells Paul, 'My grace is sufficient for thee: for my power is made perfect in weakness.' This aligns perfectly with the Particular Baptist and Reformed emphasis on the Doctrines of Grace: we are utterly incapable of sustaining ourselves in the Christian life, yet we are more than conquerors through Him who loved us. Our stability and perseverance are not the product of our own energy or resolve, but are a gift of free and undeserved favor, as noted in All of Grace Ch. 18. When we face trials, we are not promised that the struggle will disappear, but that Christ will be our anchor, enabling us to stand firm.