The covenant of circumcision (diathēkēn peritomēs). A covenant marked by (genitive) circumcision (no article) of which circumcision is the sign (Rom 4:11) as set forth in Gen 17:9-14. In the ancient Greek diathēkē was usually will (Latin, testamentum) and sunthēkē was used for covenant (sun, together, rather than dia, between). But the lxx and the N.T. use diathēkē for covenant (will in Heb 9:15.) as Lightfoot on Gal 3:16 says: “The lxx translation and New Testament writers probably preferred diathēkē as better expressing the free grace of God than sunthēkē.”
And so (kai houtōs). After the covenant was made and as a sign and seal of it.