Why does Antony repeatly refer to Brutus as an "honorable man" during the funeral speech for Caesar?

He keeps referring to him as an "honest" man because he's being sarcastic. What he says and what he means are two completely different things. He's prodding at Brutus' guilt..... trying to get under his skin. The fact that Brutus really is honorable makes the sarcasm even more potent.

Antony does not have it easy. He is already a man distrusted by the conspirators for his friendship with Caesar. Brutus lets him speak at Caesar's funeral, but only after Brutus, a great orator in his own right, has spoken first to "show the reason of our Caesar's death." Brutus makes it clear that Antony may speak whatever good he wishes of Caesar so long as he speaks no ill of the conspirators. But Antony has two advantages over Brutus: his subterfuge and his chance to have the last word. It's safe to say that Antony makes the most of his opportunity.

Antony's performance on the bully pulpit should come as no surprise. It is obvious from his Act III, sc. i meeting with the conspirators that he means something different in nearly everything he says. He even subtly mocks the senators with his lines "My credit now stands on such slippery ground/That one of two bad ways you must conceit me/Either a coward or a flatterer." Antony is the picture of disingenuous. Brutus, ignoring the more sensible misgivings of Cassius, takes Antony at his word. We, however, know what's in store when Antony in private utters, "O, pardon me, thou bleeding piece of earth/That I am meek and gentle with these butchers!"

Brutus is clearly overmatched at Caesar's funeral, both by Antony's duplicity and oration. Brutus gives a reasoned prose speech that convinces the crowd Caesar had to die. Then, for reasons that remain questionable even taking naiveté into account, Brutus not only yields to Antony but leaves the Forum altogether. Antony will expend 137 lines of blank verse before he's done, using rhetoric and calculated histrionics to incite the crowd into a mob frenzy. All quite masterful for a man who denies any ability to "stir men's blood," as he puts it.

In the speech that follows, Antony merely sets the table for dissent. He progressively hits upon the notes of ambition and honourable in a cadence that soon calls both terms into question. Antony's prime weapons at the beginning are his conspicuous ambiguity regarding Caesar ("If it were so, it was a grievous fault") and Brutus ("Yet Brutus says he was ambitious"), rhetorical questions ("Did this in Caesar seem ambitious?") and feigned intent ("I speak not to disprove what Brutus spoke"). More chilling, however, is Antony's cynical epilogue to the funeral speech as the mob departs: "Now let it work: mischief, thou art afoot/Take thou what course thou wilt!" As Antony exemplifies, the art of persuasion is not far removed in Julius Caesar from the craft of manipulation.