The salvation of mankind is the direct product of God’s election. The electing choice of God was determined prior to the foundation of the world and not bound by foreseen choices but on the basis of His purposes and grace. Yet the concept of "union with Christ" is predominantly seen in relation to an event simultaneous with conversion and rarely seen in relation to the electing purposes of God. The great Trinitarian discussion, by Paul, in Ephesians 1:3–14 begins with a call to bless God (v.3a). Reasons for the blessings are because God is the one blessing people. As God being described as the blesser, the text follows with three statements to further refine his blessing: “with all spiritual blessings,” “in the heavenly realms,” and “in Christ.” This spiritual blessing is most likely defined by the subsequent benefits of election, sanctification, redemption, being an inheritance for God, and having the Holy Spirit provide the seal for the elect (Eph 1:3–14). All spiritual elements needed are provided through God who blesses us. Moreover, the placement of these blessings is most likely given from “the heavenly realms.” That is, God, who is in heaven (locale), provides the place of disseminating these blessings. However, what is vital for the understanding how to inherit these blessings comes to fruition in the final phrase, “in Christ.” Those who are enjoined to Christ are the direct recipients of blessings of God. Only those united to Christ are the beneficiaries of God’s blessings; as Paul defines, are all spiritual blessings needed for man.
Ephesians 1:4 discuss three major elements that are intricately linked.
Just as He chose us in Him before the foundation of the world for us to be holy and blameless, appearing before him with love.
Election, union with Christ, and sanctification are closely linked in this verse. The problem many want to do is change the referent of “in Christ.”
God chose the believer for his glory and that it had to be done in connection with the redemption accomplished in Christ.[1]
That is, the election of sinners is in anticipation of what Christ will accomplish through the cross. The position sounds biblical and is viable for what follows suit in v. 7. Rather the phrase “prior to the foundation of the world” ought to dictate the time of the election in Christ. Therefore, the electing purposes of God, with our union, was prior to the foundation of the world.
To further corroborate this pre-creation union with Christ, Paul explains grace given to those united in Christ prior to the formation of ages in 2 Tim 1:9.
Who saved us and called us to a holy calling, not because of our works but because of his own purpose and grace, which he gave us in Christ Jesus before the ages began. It’s evident through the initial expression of this verse to provide a description of God’s saving and calling elements. However, this calling and salvation was not based on any foreseen work of effort to gain favor with God but on the basis of God’s purpose and grace. Both purpose and grace are in the feminine gender followed by an articular singular passive participle “give.” Therefore, it is a reference to the nearest antecedent, grace. This grace, then, was given to those united in Christ prior to the eternal ages, a reference prior to creation.
The implications of the electing purposes of God and its association with “union with Christ” are incredible. First, it assumes the presence of the trinity prior to creation and the eternal existence of Christ. It provides confidence in the electing purposes of God prior to any human response, thereby confirming confidence in Evangelism. Moreover, current believers were the recipients of God’s grace and calling before time. And finally, those that were called to salvation and given grace were previously united to Christ. One of the implications I don't observe the texts discussing is, those united to Christ prior to creation have Christ as a federal head. The enjoined being discussed promises and guarantees the recipients grace and spiritual blessings of election.
[1] Hoehner, Ephesians, 177.