Romans 8:28–30 is as good as a mathematical equation, the sum of which is the Doctrines of grace. The equation tells us that all whom God calls will certainly be with him in glory after the judgment, and that their calling was ordained in eternity past.
The elect are irresistibly called by God at the appointed time as a result of His foreknowing and predestining in eternity past. This effectual summons is rooted and grounded in the eternal purpose of God:And we know that for those who love God all things work together for good, for those who are called according to his purpose. For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified. —Romans 8:28–30
All whom the Father foreknew before the foundation of the world—that is, those He foreloved and foreordained—are effectually called within time by the Holy Spirit to come to Christ. Those the Father foreknew and chose He predestined to believe and be saved. And all those who were predestined are irresistibly summoned to believe in Christ. Hodge explains, “Every time predestination is spoken about it is clear that the reason for the choice does not lie in us. We are chosen in Christ according to the free purpose of God, etc. This is a fore-ordination, a determination which existed in the divine mind long before the event and even before the foundation of the word (Ephesians 1:4). When it occurs in time it shows the eternal purpose of God, and it executes the plan of which it forms a part.” That is to say, all those foreknown by God are called by God.
—Steve Lawson, Foundations of Grace (Reformation Trust, 2006), 368.