1 Corinthians 15:28

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

Then: At the end, when Jesus Christ the Son has victoriously conquered all, he will himself be subject to God. Paul gives the Corinthians a picture of the entire church coming forward into God's presence. In his presentation, Paul does not bring up the aspect of mutual submission. He is interested in explaining that in the final order of the universe, the church as the body of Christ, will be brought forward to God the Father through the one who has authority over it. From then on God will be "all in all." N. T. Wright emphasizes that this passage follows the first understanding of "order" as chronological events and concentrates on showing "order" as divine purpose, tasks, and relationships.

Now: In this passage Paul, does not remind the Corinthians about the on-going eternal relationship of God, the Father, with Christ the Son (Baker, p. 224). Paul's purpose is to show the completion of Christ's messianic mission. Paul never actually talks about God as being three in one. But as theologians study his letters, they understand his intent and description to mean one God in three persons.

Extra Source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.