All people are descendants of Adam (Acts 17:26). When Adam disobeyed the Lord, he lost access to the tree of life, which would have enabled him to have lived an indefinite mortal life (Gen 3:22-23). This broken access from the presence of the Lord resulted in the eventual physical death of Adam (Gen 5:5).
All descendants of Adam therefore live imperfect, limited mortal lives. Separate from the presence of the Lord, all mortal men continue to commit offenses against both the Lord and their fellow human beings (Rom 3:10 and Rom 3:23). Although himself mortal, Jesus was the incarnation of eternal life, which was from heaven (John 1:1-3 and 1 John 1:1-3). He committed therefore no wrong against either the Lord or against his fellow human beings (2 Cor 5:21).
As such, he was qualified to be condemned for sins, since he had never been condemned by any sin of his own (2 Cor 5:21). Thus the Lord made him to be sin, and therefore he was separated from the presence of the Lord and therefore died (Mark 15:34). But because his eternal life was indestructible (Heb 7:16), death was not capable of containing him (Acts 2:24). In this respect, it was not death that had destroyed life, but eternal life that had abolished death through resurrection (2 Tim 1:10).
In this sense, Jesus reversed the condemnation incurred by Adam, and so Jesus now appears as another (second) Adam who undid or reversed what the first Adam committed. The graph, below, provides the illustration.
Please click here for the full discussion and explanation of this graph in detail.
Therefore those who believe on Jesus lose their condemnation and mortality in Adam, and share the justification and immortality of Jesus, who conquered death.
Thus the "tie in" to Jesus. Those who believe in him are no longer united to the first Adam (condemned and shamed), but are united to the second Adam (justified and glorified). Since the second Adam is gloried in heaven at the right hand of the Father, believers, too, share the positional glorification at the right hand of the Father as well (Rom 8:30). Thus believers are "seated with Him in the heavenly realms."