Moshe Rabbeinu was placed in jeopardy on his way back to Egypt for not having yet circumcised his son. Aside from the timing of this incident, its narrative is unusual, as there is no break (piska) between this story and the previous text in which Hashem told Moshe to convey to Pharaoh, "...Send out My son in order that he may serve Me; and - if you refuse to listen - behold, I will kill your firstborn son." (Shmos 4:23). Why is the circumcision issue especially relevant upon Moshe's return to Mitzrayim? Why is its text not set apart from the prior instruction of God to tell Pharaoh to set the Jews free?

Milah is a very unique mitzvah insomuch as the orlah represents a division between God and Man, and milah thus allows Man to stand before God and relate to Him in a closer fashion. It is sort of a spiritual redemption of Man, enabling him to unite with his Creator. (Milah is also viewed as a completion of the human body, perfecting it according to God's Will.) Thus, since Moshe Rabbeinu was on his way to carrying out a mission of physical and spiritual redemption of his people, it was unconscionable that his own son was in an "unredeemed" state. Therefore, milah was very apropos.

This also explains the extreme juxtaposition of milah and Moshe's statement to Pharaoh to set the Jews free to serve Hashem; for milah is a source of spiritual connection to God and His service, and the upcoming Jewish spiritual liberation was identical in theme.