Union with Christ, isn't a mystical state you reach after mediating or something that occurs during the Lord's Supper or a particular stage in salvation. Instead it's more of a category that runs through whole Ordo Salutis, a way of considering how we are connected to Jesus in each of the separate aspects of Salvation. This is why I've shown it as a red arrow in the slide below rather than a symbol.
accessible little summary gives you a sense of how often the concept appears) through the different doctrines of grace it's hard to name a single representative or summative verse. Essentially it's our spiritual connection to the person of Jesus that matters because it's by his work we are saved. It's not that we become absorbed into God in some sort of Buddhist eternity sense or that it's only a token connection, like being a member of the Wilderness Society but as a spiritual aspect of our identity which of course as Christians is dominated by the saving work of Jesus.
Clearly what this doctrine means is that we don't blunder through salvation like a runner on an orienteering course, marking-off our checkpoints saying: "Predestination, done, now on to Justification, wave to Jesus on the way, (I'm glad he made this course, that I now have to run by myself!)." No, if predestination is God at work through all history and regeneration is the Holy Spirit at work behind the scenes and Sanctification is God making us holy, Union with Christ keeps us close to Jesus throughout the whole process.