What does it mean to be eternally united to Christ?
Eternally: The comfort of glorification is that it is eternal, that is, non-revocable. God began our salvation process, he is carrying it out and he will complete it to his own honor and glory. What God has begun, he will accomplish. He who began a good work in you will carry it through to the day of Christ Jesus. “So we do not lose heart. Though our outer self is wasting away, our inner self is being renewed day by day. For this light momentary affliction is preparing for us an eternal weight of glory beyond all comparison, as we look not to the things that are seen but to the things that are unseen. For the things that are seen are transient, but the things that are unseen are eternal. For we know that if the tent that is our earthly home is destroyed, we have a building from God, a house not made with hands, eternal in the heavens.” (2 Corinthians 4:16-5:1 ESV)
Certainty: He uses the word in 8:30 as if it has already been accomplished. Our glorification is as good as done.