The poem with which Winthrop ends his famous sermon "A Model of Christian Charity" is a summary of Deuteronomy 30:19-20. In it, Moses enjoins the people of Israel to keep the terms of their covenant with God; their possession of the Promised Land depends on it. This is what Moses means when he says that the Israelites must choose life, so that they and their children will live. Their occupation of the Promised Land, and that of their ancestors, is conditional upon their obedience to the Law.
For Winthrop, this particular passage of the Bible is especially relevant to the message he is preaching to his fellow Puritans. They too are about to take possession of the Promised Land—in this case, America—where they intend to establish a godly commonwealth.
Just as Moses enjoined the people of Israel to keep God's Commandments and Laws, so Winthrop drives home a similar message to his hearers. They have also entered into a sacred compact with God, which they must obey if they are to thrive and prosper in their new homeland. If not, then like the people of Israel, the Puritans will suffer the consequences of turning their hearts from God. Or, as Winthrop puts it,
We shall surely perish out of the good land whither we pass over this vast sea to possess it.