In the "Teaching Tips for this week's lesson Greg Blake writes:
Verses 14-18 are widely recognized as a hymn of praise that the apostle inserted into his text. The “for” that marks the beginning of this section and the “now therefore” that signals its end in verse 19 clearly distinguish it from the rest of the surrounding texts. The overarching theme of this hymn is introduced in verse 14 — “he is our peace.” The peace Christ has brought is of two kinds. It is a peace between Gentiles and Jews (vs. 14, 15). Yet even more remarkable, it is a peace between all humanity and God.
Source: Galatians and Ephesians: Adult Teacher's Insights, pp. 57-58.