Deuteronomy 26:1
And it shall be, when thou art come in unto the land which the LORD thy God giveth thee for an inheritance, and possessest it, and dwellest therein; And it shall be, when thou art come in unto the land which the LORD thy God giveth thee for an inheritance, and possessest it, and dwellest therein;
The verse fixes the time for all that follows as after coming into the land the Lord gives for an inheritance, taking possession of it, and dwelling or settling in it. The land itself and its blessings are God's gift to Israel as an inheritance. It therefore opens the duties appointed for life in the land, by which the gift was to be acknowledged with gratitude and obedience, including bringing the firstfruits and keeping His commands.
These directions are God's law for His people, spoken by Christ from the pillar of cloud, and given in plain terms so that they would not be left uncertain or forgetful. Their commands teach that obedience is for present and future good and brings prosperity, while disobedience brings a curse, and that those whom God has specially blessed are to acknowledge Him publicly.
The firstfruits were to be set apart and consecrated to God as an offering of gratitude acknowledging His goodness. That gratitude looked back to His mighty acts in delivering Israel from Egypt and bringing them into a good land. The offering was acceptable only when presented with reverence and gratitude, and to bring it in self-sufficiency, as though God were indebted to the giver, made it unacceptable.