Pentecost is about God keeping His promises, to every generation of His people. Peter's sermon includes promises inspired by God and recorded by David and Joel centuries before. God will keep His promises to them. Peter was inspired to preach that "the promise is to you and to your children, and to all who are afar off, as many as the Lord our God will call." Pentecost is for our loved ones who have died, and for our children and grandchildren, too. God is keeping His promises.