Ver. 9.—That ye may prosper in all that ye do. The verb here used (הִשְׂכִּיל) means primarily to look at, to consider or attend to, hence to become intelligent, to be prudent, to act wisely, and so to have success, to prosper. It is the prosperity which comes from wise and prudent action that God commends to his people (cf Josh. 1:7, 8).
29:9 keep the words of this covenant. The spiritual experience of God’s faithfulness to Israel should have led to obedience to the stipulations of the Sinaitic Covenant in the future, but could not without a transformed heart (vv. 4, 18) and the true knowledge of God (v. 6).
29:9 In light of all of God’s mighty acts on Israel’s behalf (vv. 5–8), he had every right to expect them to observe the Deuteronomic covenant and thus ensure their success; but human nature being what it is, even miraculous displays of power did not always lead to faith and repentance (Jn 12:37).
 MacArthur, J. F., Jr. (2006). The MacArthur study Bible: New American Standard Bible. (Dt 29:9). Nashville, TN: Thomas Nelson Publishers.