What is the purpose of a counterargument in a persuasive text?

Which lines from the speech best supports this topic sentence? and as we walk, we must make the pledge that we shall always march ahead. we cannot turn back. there are those who are asking the devotees of civil rights, "when will you be satisfied? " and they have come to realize that their freedom is inextricably bound to our freedom. we cannot walk alone. we refuse to believe that there are insufficient funds in the great vaults of opportunity of this nation. and so we've come to cash this check, a check that will give us upon demand the riches of freedom and the security of justice. and so let freedom ring from the prodigious hilltops of new hampshire. let freedom ring from the mighty mountains of new york. let freedom ring from the heightening alleghenies of pennsylvania.