Knowledge of God does not always translate into actions based on that knowledge.
The following article is reprinted with permission from Kolel: The Adult Centre for Liberal Jewish Learning.
Moshe continues to review the history of the Israelites from the time of their liberation from Egypt; he also repeatedly implores them to accept and faithfully follow the Torah, stressing its goodness and wisdom. Moshe warns the people not to worship any other god or power except the One God who gave them the Torah. Moshe then reiterates the Ten Commandments. The paragraph which we know as the first paragraph of the Shma forms part of Moshe's exhortation to the people to keep faith with God after they enter the land, when Moshe himself will no longer be able to guide or instruct them.
"Know this day and set it upon your heart that Adonai is God--in heaven above and on earth below--there is no other." (Deuteronomy 4:39)
Moshe delivers a long sermon to the people on the dangers of forgetting their experience of Liberation and Revelation--he warns them that they might fall into idolatry once they enter the land of Israel. He also promises that God will take them back, just as God took them out of Egypt to be a unique people. Moshe urges the people to remember the giving of the Torah at Sinai and be mindful of God's presence.
The main point of Moshe's sermon seems fairly straightforward: don't forget about the God who liberated you once you settle in the Land. The verse above could therefore be a simple rhetorical device, employing extra phrases merely for emphasis of the basic point.
Read this way, there would be no substantial difference between "know this day" and "set it upon your heart"--they might mean basically the same thing, a steady consciousness of God's existence, authority, and instructions. The next phrase, "in heaven above and on earth below," could also be read this way, as complementary images which strengthen each other. Scholars of Biblical rhetoric and poetry call this "parallelism," from the idea that two parallel or similar images strengthen the rhetorical point but don't really have two different meanings in themselves.
Traditional rabbinic Bible commentators, on the other hand, often like to read the text in more expansive and creative ways, perceiving new and additional meanings in each seemingly superfluous word. Thus Rabbi Israel Lipkin of Salant [popularly known as R. Israel Salanter], a 19th century giant of mussar [ethical development] teachings, sees "know this day" and "set it upon your heart" as two different stages in a process: