If we were to ask the Apostle Paul what was the most important concept in his theology, I don't think we would get the answer "grace" or "mercy". I think he would say "Love". I am not saying that grace and mercy are not important to Paul's theology, they are indeed but I think Paul would know that behind those two concepts I just mentioned is God's love. God is gracious and merciful because he is love.
I want to reflect upon Paul's most densely "loving" Epistle, Ephesians. This is an interesting book for several reasons. Firstly, the fact that in most manuscripts "To the saints in Ephesus" do not appear, together with the lack of personal greetings at the end has made some scholars suggest that this is perhaps a "general letter" which would have been read in various cities; perhaps the seven churches in the area mentioned in Revelation. Also, Paul is not dealing with any are of church belief, as in Galatians or of church life, such as in the Corinthian correspondence but general themes. Finally, this seems to me to be Paul's mature theology. It was probably written around about AD60 and the time of the Colossian Epistle and so by no means his final letter but the themes are the grand themes of salvation history, redemption, unity in Christ and how our individual lives are to be lived to fulfill those great plans.
The first three chapters speak of God's love for us and the world. In love God predestined us for Sonship (1:4); he has made us alive because of his love (2:4-5); we can only know the unknowable riches of God's love when we are rooted and established in love (3:17-18). God is calling out, choosing, redeeming and establishing a people for his purpose. Ephesians 1:10 tell us that this purpose is to "bring all things in heaven and earth under Christ".
The last three chapters focus upon the love that we should have for others to bring those purposes about. We must bear one another in love (4:2); we should speak the truth in love (4:15); the body, the church builds itself up in love (4:16). In chapter 5 we are to follow Christ's example (5:1) and live a life of love (5:2); the marriage relationship is to be marked by love (5:25-33). This may seem obvious but marriage was not always romantic! Finally, Paul greets the Ephesians with the love of God in love (6:23-24).
This gran purpose of God has its origin in God's love for his creation; he chooses a people out of love; he saves them because of his love; he establishes them in love and unifies them in love. Then as they live out that unity in love and in all their loving relationships, God achieves his loving purposes.