The argument deserves to be stated in its strongest form. Classical Christians do not usually mean that the Incarnation made God less than God, or that the Son ceased to possess divine glory. They mean that the eternal Son freely assumed a complete human life and, within that assumed condition, accepted poverty, obedience, suffering, and death for the salvation of others. On this reading, the Incarnation is not a divine defect but a divine perfection displayed in history: the Lord of all stoops to serve because self-giving love is not beneath greatness but belongs to greatness.
The central text is Philippians 2:5-11. Paul urges believers to have the mind of Christ Jesus, and the passage is genuinely strong evidence for the classical Christian claim: Christ is portrayed as preexistent, "in the form of God," yet choosing the path of self-emptying and obedience rather than grasping at status. In a close rendering:
"who, being in the form of God, did not consider equality with God something to be used for his own advantage, but emptied himself, taking the form of a slave, being born in human likeness; and being found in appearance as a human being, he humbled himself, becoming obedient to death--even death on a cross. Therefore God highly exalted him and gave him the name above every name, so that at the name of Jesus every knee should bend ... and every tongue confess that Jesus Christ is Lord, to the glory of God the Father."
Important details should be handled carefully. The Greek morphe theou ("form of God") is not a simple claim that Jesus merely looked divine, nor does ekenosen ("emptied himself") mean that the Son stopped being divine. In context, the "emptying" is explained by the participles that follow: taking the form of a slave, being born in human likeness, and obeying unto death. At the same time, the passage distinguishes Jesus from "God the Father" and says God exalted him, so it does not by itself settle every later Trinitarian question. It does, however, give Christians a serious biblical basis for seeing humility, service, and incarnation as compatible with divine glory rather than as a denial of it.