And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.
Son also himself be subject - not as creatures are, but as a Son, voluntarily subordinate to, though co-equal with the Father. In the mediatorial economy the Son had been in a manner distinct from the Father; Now His kingdom shall merge in the Father's, with whom He is one: not that there is any derogation from His honour; for the Father wills "that all should honour the Son, even as they honour the Father" (John 5:22-23; Hebrews 1:6).
God ... all in all - as Christ is all in all (Colossians 3:11: cf. Zechariah 14:9). Then and not until then, "all things" shall be subject to the Son, and the Son subordinate to the Father while co-equally sharing His glory. Contrast Psalms 10:4; Psalms 14:1. Even, the saints do not fully realize God as their "all" (Psalms 73:25) now, though desiring it; then each shall feel, God is my all.