"From the very beginning (from eternity itself), there are no other elect together with or apart from Him, but, as Eph. 1:4 tells us, only 'in' Him. 'In Him' does not simply mean with Him, together with Him, in His company. Nor does it mean only through Him, by means of that which He as elected [human] can be and do for them. 'In Him' means in His person, in His will, in His own divine choice, in the basic decision of God which He fulfills over against every [human]. What singles Him out from the rest of the elect, and yet also, and for the first time, unites Him with them, is the fact that as elected [human] He is also the electing God, electing them in His own humanity. In that He (as God) wills Himself (as [human]), He also wills them. And so they are elect 'in Him,' in and with His own election."
Karl Barth, Church Dogmatics II.2: The Doctrine of God, Study Edition. Edited by G.W. Bromiley and T.F. Torrance. Translated by G.W. Bromiley (London: T&T Clark, 2009), 124