You affirm that the testimony you're about to give before the committee will be the truth, the whole truth--
What Trump has done is fire the head of the FBI for a reason that nobody in Washington believes-- namely that he behaved unprofessionally last year in the handling of the investigation into Hillary Clinton's email scandal. And nobody believes that reason, because of course Trump was the biggest beneficiary of this investigation.
Comey was due to testify on Capitol Hill this coming Thursday. This came 24 hours after Sally Yates, the former acting Attorney General who Trump also fired, had testified to Congress about the circumstances surrounding her dismissal, five days after Trump was inaugurated. The timing of this would at any point be very, very dramatic. It pretty much guarantees that suspicions about Trump's motives and about Russia's alleged collusion with his campaign will dominate the news cycle, really for the foreseeable future.
The second paragraph of what is a three-paragraph letter is a quite extraordinary wording. To any casual observer, it would belie Trump's obsession with alleged Russia collusion.
Trump's going to have to find somebody who's going to get at least 50 votes in the Senate to be confirmed as the new head of the FBI. The Democrats will be looking for three Republican votes to deny whoever Trump nominates to replace Comey. So they will be disinclined to confirm anybody who isn't fiercely independent in their reputation.
Meanwhile, there is mounting pressure on Trump to appoint a special independent prosecutor to investigate these claims. The potential for this to metastasize into an all-consuming Washington crisis that really dominates the agenda and stops Trump's from dealing with anything else is pretty big right now. The only previous time something comparable has happened was when Nixon, in October 1973, fired Archibald Cox in the so-called "Saturday Night Massacre." Nine months later, Nixon is out of a job.
To fire the person who's leading the investigation potentially into the president is-- the only precedent for this is what Nixon did, but even that really isn't a good precedent. This is a unique situation and it's got potentially huge implications.