A.T. Robertson Commentary 1 Corinthians 15:28

"And when all things have been subjected unto him, then shall the Son also himself be subjected to him that did subject all things unto him, that God may be all in all." — 1 Corinthians 15:28 (ASV)

And when all things have been subjected (οταν δε υποταγη τα παντα). Second aorist passive subjunctive of υποτασσω, not perfect. Merely, "when the all things are subjected unto him." The aorist subjunctive has given translators a deal of needless trouble in this passage. It is prophecy, of course.

That God may be all in all (ινα η ο θεος παντα εν πασιν). The final goal of all God's redemptive plans as Paul has so well said in Ro 11:36. Precisely this language Paul will use of Christ (Colossians 3:11).