Regeneration is a secret act of God in which he imparts new spiritual life to us. As the gospel comes to us, God speaks through it to summon us to himself (effective calling) and to give us new spiritual life (regeneration) so that we are enabled to respond in faith.
Effective calling is thus God that Father speaking powerfully to us, and regeneration is God that Father and God the Holy Spirit working powerfully in us, to make us alive. Sometimes the term irresistible grace is used in this connection. It refers to the fact that God effectively calls people and also gives them regeneration, and both actions guarantee that we will respond in saving faith.
The term irresistible grace is subject to misunderstanding, however, since it seems to imply that people do not make a voluntary choice in responding to the gospel - a wrong idea, and a wrong understanding of the term irresistible grace. The term does preserve something valuable, however, because it indicates that God's work reaches into our hearts to bring about a response that is absolutely certain - even tough we respond voluntarily. - Wayne Grudem from Systematic Theology (pg. 699)