In chapter 11 of St. Paul's Letter to the Romans he deals with God's present relationship with the Jews. There were at the time some who thought that the "new and everlasting covenant" of which we Christians are a part meant that God had abandoned the covenant that he made with Abraham. St. Paul's response is an unequivocal no.
He then goes on to explain one of the many paradoxes in the Bible. As he explains it, the people of Israel "transgression." There are several words for sin in the New Testament. The one St. Paul uses here, paraptoma, means to slip and fall, or to fall away.
Rather than simply punishing them for their transgression, God used their transgression for something miraculous. In his words,
their transgression is enrichment for the world
God knows his creatures, he knows how quickly we react when we are jealous. And so, according to St. Paul,
through their transgression salvation has come to the Gentiles, so as to make them jealous.
Remember when we were children. We may have had a particular toy which we no longer about at al. But let someone else start playing with it and suddenly "That's mine. I want to play with that."
In St. Paul's understand of God, God loves us so much that he will use whatever it takes to bring us back. In the hands of God, anything, even darkness, can be transformed into an instrument of salvation.