The last will be first, the first last. Jesus opened the kingdom to all—to gentiles, publicans, and sinners. He opened the kingdom to all who’d simply answer in faith and penance.
He didn’t count heritage or human achievement, only faith. It didn’t matter if you were descendent of Abraham or learned or ritually pure. You just needed to fall in love with Jesus, to follow him. That’s what was so challenging and new, which some got right away and others not. It’s why some were first and others last.
And it’s a lesson for us, not to count so much on our heritage or religious achievement. To be honest, God doesn’t care that you’re a cradle Catholic. He doesn’t care if you went to Catholic school. A blessed heritage all of that is, certainly, but it means nothing if you haven’t fallen in love with Christ, if you haven’t given your heart to Jesus. If you take comfort or pride in being Catholic, but you don’t have a personal relationship with Jesus, then this parable from Matthew—his warning that the first will be last and the last first—ought to bother you. Because it foretells a lesson you will one day learn: that what you think matters really doesn’t.
What matters is Christ: his love for you and your love for him. Our Catholic faith is beautiful, but the quickest way to hollow it out and make it meaningless is to think being Catholic means you don’t need to have a personal relationship with Jesus. Catholicism is the fullness of Christianity only because it brings you into contact with the fullness of Jesus. It has no meaningful value as a religion aside from that. That’s the deeper lesson of the gospel, that it really is all about Jesus.