What effect does the remark, "As I am sure they do" have on Brutus's speech in Act 2 of Julius Caesar?

The line you ask about in Act 2:1 of Shakespeare's
Julius Caesar doesn't relate to Brutus trying to get the
conspirators on his side--the deal has already been done by this time.  All this speech
by Brutus is about is whether or not they should all swear an
oath. 

The speech starts in line
121:

No, not
an oath.  If not the face of men,

The sufferance of our
souls, the time's abuse--

If these be motives weak, break
off betimes,

And every man hence to his idle
bed.

So let high-sighted tyranny range
on

Till each man drop by lottery [be killed on the whim of
a dictator].  But if these

(As I am sure they do) bear fire
enough

To kindle cowards and to steel with
valor

The melting spirits of women, then,
countrymen,

What need we any spur but our own
cause

To prick us to redress?  What other
bond

Than secret Romans that have spoke the
word

And will not palter [waver, shift
position]?...

Brutus simply
says that an oath is not necessary, that the face of the men involved in the deal, the
suffering of those men's souls, and the abuse of the times (presumably under Caesar)
binds them together and will make them go through with the assassination.  An oath is
not necessary. 

In fact, the line you ask
about:

(As I
am sure they do) (line
131)

goes back to lines 125
and 126.  The these, in line 130, that leads up to the "(As I am
sure they do)" refers to the faces, souls, and abuses of 125 and 126. 
These is a pronoun that refers back to that which will hold the
conspirators together, and Brutus is giving the conspirators a vote of confidence.  He
is sure these conspirators possess enough fire and valor to continue on with their
plan--no oath is necessary.