What is Reformed Theology/Calvinism 9: Irresistible or Effectual Grace- God Acts 1st

If everyone agrees that grace is resisted by all men, then why do Calvinists talk about irresistible grace? Is grace alone sufficient to save? Does our consent & faith come from grace or do we get grace because we believe? Is grace effectual? Does regeneration precede faith? Does God respond to our faith with the gift of regeneration/new birth? What is grace? Is grace necessary to believe? Why does fallen men need grace before coming to faith? Is faith a gift of God or an act of man?