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.
In my opinion, the major impact of this remark is to help get the conspirators onto Brutus's side. It is a very persuasive thing to do, a good rhetorical thing. He is saying that if they have good reasons for wanting to kill Caesar, then they have what they need and they should just get with it and do it already.
While saying this, he drops in this part about how he is sure that they do have these reasons. So he is inviting his listeners to agree with him. He is inviting them to feel like they are in this with him and that there really is no question as to whether they have good reasons to proceed with their plot.