The term for “made subject” (hupotageesetai) is used in contexts where the meaning can only be position and rank (not in essence or nature). This term stems from the verb hupotasso, which is used quite often in Scripture as in the following examples:
Christ made Himself subject to His parents by virtue of His sonship. He obviously wasn’t inferior in nature to them in any way, since He is the Divine Savior who had come to save them and others from their sin (cf. Matthew 1:21). Thus, the subjection is purely one of authority and position.
Christians are commanded to submit to the governing authorities, to human rulers. This again shows that the verb relates to a subordination of authority or rank, not of essence or nature.
Christians are also commanded to submit to one another:
It is only at the end of the age that Christ will then assume this subordinate role. But for now He is equal even in authority to the Father. And even after assuming the subordinate role, He will still be equal with God in nature.
The term “subject” (or “submit”) does not necessarily imply that the person that is submitting is inferior in nature and essence to the other. It can simply refer to a person being in a lesser position of authority including a voluntary submission to somebody who has no natural or enforceable position of authority, like one Christian to another.
Moreover, the context of 1 Corinthians 15:28 indicates that the Greek passive hupotageesetai should be taken as a middle voice. This would mean that the Son is not being made subject, or that someone is forcing Him to submit. Rather, the middle voice suggests that the Lord Jesus is voluntarily subjecting Himself to the Father. As Greek NT scholar Spiros Zodhiates noted:
Thus, Christ’s submission is a voluntary act intended to bring perfect unity and peace between God and all creation, the very point Paul was making in context:
Further, to draw this back to a text we were on a little earlier (Ps. 2:7), in Ps. 2, the rulers that were brought under subjection, does this mean they lost their status as earthly rulers?