But he echoed the prime minister in calling the agreement a "decisive" step and suggested he could now call a summit of European Union leaders to consider approving it.
Asked whether she was prepared to risk a split in her government as divisions in her party mount, May said that she felt MPs would honor the will of the British people when it came down to a vote.
Speaking outside Number 10, the Prime Minister said: "I firmly believe that the draft withdrawal agreement was the best that could be negotiated and it was for the Cabinet to decide whether to move on in the talks".
And leading backbench Brexiteer Jacob Rees-Mogg submitted a letter of no confidence in Mrs May to Sir Graham Brady, chairman of the Tories' backbench 1922 Committee.
So far MPs in the East of England have held their tongues as they wait for more detail over the Brexit agreement.
Raab announced his resignation on Thursday morning with Work and Pensions Secretary Esther McVey following suit later in the morning.
Saying her deal is "in national interest" and calls on country to "unite behind" the deal. "Ultimately, you deserve a Brexit secretary who can make the case for a deal you are pursuing with conviction".
Mrs May spent almost three hours fielding largely critical questions from MPs before holding a press conference in Downing Street to further answer her critics. "And there's been much criticism in that time of the government's approach", May said.
The government, for today at least, is at the mercy of events not in control.
"I can not in good conscience support the terms proposed for our deal with the EU", Raab said in a tweet.
Two Leave-backing ministers including the man in charge of the UK's withdrawal negotiations have sensationally quit the Cabinet in a massive blow to Theresa May's Brexit plans. "I think MPs across my party who look at that deal will recognise the importance of delivering on the vote of the British people and recognise the importance of doing that in a way that does protect people's jobs, protect security and protect the unity of our United Kingdom".