But God, being rich in mercy, because of the great love with which he loved us,  even when we were dead in our trespasses, made us alive together with Christ—by grace you have been saved – Eph. 2:4-5
The riches of God that Paul refers to when he says that God is rich in mercy are wonderful and divine. In this world a rich person uses his wealth for his own good and comfort. Even if he gives some of it away to help others, he uses most of it for his own benefit. In contrast, the kind of wealth in which God is rich in mercy is not intended for his benefit but for ours. Mercy cannot benefit him in any way because he is not miserable, unhappy or in trouble and therefore does not need it. All the wealth that makes him rich in mercy is used up outside himself and flows into us rather like the waters of a gushing fountain. Such riches reflect the divine nature, whose attribute is always to have mercy and to pardon. In the world nobody gets rich by showing pity, giving, contributing and donating, but only by obtaining things. That is what the riches of this world are like. Let us therefore leave them to the children of this world and let ourselves be rich in mercy, following the example of our Father in heaven.
Reformation Commentary on Scripture: Galatians, Ephesians (Downers Grove, IL; IVP Academic; 2012) p. 281-282.