When Paul says that his readers are rooted and founded in love (Ephesians 3:17), he is not speaking about an abstract concept or a general feeling. Rather, his readers have been rooted and founded in love because they have been incorporated into the one body of believers that includes both Jews and Gentiles.
According to Paul, it was God’s love that put the plan into motion to bring all things together in Christ Jesus (Ephesians 1:4b-10). Among the “all things” being brought together in Christ is humanity itself. Paul reminds his Gentile readers that apart from Christ they were:
alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility by abolishing the law of commandments expressed in ordinances, that he might create in himself one new man in place of the two, so making peace, and might reconcile us both to God in one body through the cross, thereby killing the hostility.” (Ephesians 2:12-16)
In the church in which Jews and Gentiles were learning to live together through faith in Christ, love was more than just an emotion or a state of mind. Love was the foundation of their daily lives together. Through the cross of Christ, God was breaking down the dividing wall (Ephseians 2:14) and “killing the hostility” (Ephesians 2:16) that existed between God’s covenant people and the rest of humankind.
When Paul gives thanks, then, for the love that his readers have for ALL the saints (Ephesians 1:15), he does so in part because their love, lived out within the community of faith, was evidence that God’s eternal plan was coming to fruition in their midst.