If Christ already dwells in believers by His Spirit, why does Paul pray for strength for us so that Christ may dwell in our hearts through faith (Eph. 3:16–17)?
When we trust Christ as our Lord and Savior, He takes up residence in us by His Spirit. He indwells every true follower of Christ. In fact, if the Spirit of God does not indwell a person, that person is not in relationship with God (Rom. 8:9). But as you noted, if Christ is already in us, why does Paul pray for Christ to dwell in our hearts? The answer is found in the meaning of dwell in this context.
Ephesians 3:17 uses the word dwell to convey the idea of being at home. It is not the permanent indwelling of Christ in us that is under discussion here, but the idea of the Lord Jesus being at home in our hearts. We may live in a house permanently and still not feel at home. Sometimes we may have
guests over to our houses, and we say to them, “Make yourself at home!” The Lord Jesus does not want merely to indwell us; He wants to be at home in our hearts! Jesus wants to be at the very center of our innermost world, at the center of our being, our will, our emotions, and our thinking!