I believe that God's grace is both resistable and irresistable. I believe that grace becomes irresistable to us once we see that God is fully sovereign, completely good, holy and righteous and we are not, because He has opened our eyes, but our sins and our pride will keep us blind if we choose them, making it possible for us to resist Him.
I believe that once we come to a point of conviction that we cannot do or be anything without Him, God's grace becomes irresistable to us, because we feel Him so strongly and we know that we need Him. But up until that point, we can resist it, and I believe that most of us do. We can grieve the Spirit if we choose sin, and He is the Spirit of Grace. Ephesians 4:30, Hebrews 10:29 The flesh is at constant war against the Spirit, and if we choose to walk in the flesh, we are resisting God and by extension God's grace, because we do not want to change.
Paul's thorn in the flesh was there to buffet him so that he would be made weak, because God's grace is made perfect in weakness. 2 Corinthians 12:9. It is still our choice whether to submit to God or to keep sinning, and whether to keep walking with God or to leave the faith, which many do. God knows who are His, who will accept His grace and who will keep running the race until the end, and He has chosen each one of us who will and He won't let us be tempted beyond what we can handle in Him, or let us fall away. John 10:27-28, Matthew 24:13, Ephesians 1:4-5, 1 Corinthians 10:13. But God's grace is only irresistable to someone who knows that they need it, someone who sees their own sins, and who accepts Jesus as their Savior. Luke 18:9-14
I therefore believe that God has pre-ordained those who He foreknew would hear His call and accept His grace, and that He has planned our lives and the work we will do for Him from the foundation of the world. God has offered His grace to everyone, John 3:16, and we don't know who are chosen until they become saved, only He knows. Many are called, but few are chosen. Matthew 22:11-14 I believe that this parable represents the ones who have been called, but have not accepted God's grace, but kept living in sin. (Because accepting God's grace means to put on His righteousness Revelation 19:8, Ephesians 4:24, Isaiah 61:10 etc)
I think that may have come out a little incoherent. I'm at work, sorry.