John Murray was a 20th century biblical theologian. Consider his reflection on Ephesians 1:3-4, that we were chosen “in Christ” before the foundation of world, and its implication for union with Christ.
The Father elected from eternity, but he elected in Christ. We are not able to understand all that is involved, but the fact is plain enough that there was no election of the Father in eternity apart from Christ. And that means that those who will be saved were not even contemplated by the Father in the ultimate counsel of his predestinating love apart from union with Christ–they were chosen in Christ. As far back as we can go in tracing salvation to its fountain we find “union with Christ”; it is not something tacked on; it is there from the outset (John Murray, Redemption: Accomplished and Applied, 162).
Your union with Christ does not ebb and flow. Union is not conditional on anything finite. But rather, the only thing your union with Christ is conditional on is the Father’s eternal, electing love of having chosen you in Christ.