The DUP leader Arlene Foster has described nationalism as "narrow and exclusive" at a conference in London.
Speaking to the Union and Unionism conference, Mrs Foster said unionism stands for pluralism and multi-culturalism.
She described it as "inclusive and welcoming" and urged unionists to reclaim citizenship and rights issues.
But Sinn Fein's Máirtín Ó Muilleoir said her speech had "no new thinking but more of the same old arrogance".
Mrs Foster said the best way to cement the union is for Northern Ireland to be an open and successful place.
"A Northern Ireland which embraces differing cultures and where minorities feel valued is one that few will choose to abandon," she said.
On Brexit, Mrs Foster repeated her party's red line, insisting that it will not support a legal protocol that creates a new regulatory border down the Irish Sea.
The former first minister said she saw no logic or rationale for the creation of a hard border between Northern Ireland and the Republic of Ireland.
She accused opponents of stirring myths about border checkpoints in order to unpick the Union:
"They seek to use such imagery to advance and build support for their long-term political objective. They will not succeed."
Mrs Foster claimed that some people had sought to use Brexit as a way to foment division in Northern Ireland.
"The overwhelming majority of people in Northern Ireland understand and appreciate that they are better off being part of the United Kingdom," she said.
South Belfast MLA Máirtín Ó Muilleoir described Mrs Foster's speech as "a missed opportunity" and said it had produced "no new thinking but more of the same old arrogance".
"Where is the multi-culturalism for Gaelic speakers who are still being denied equal language rights by the DUP?" he asked.