One word--no. Having an agent is a big first step; it means someone who knows a thing or two has seen merit in the manuscript and thinks he or she can sell it. But their judgement isn't infallible, no more than yours or mine. Having a good agent will get your manuscript in the door in places where it otherwise would not, and a GOOD agent will fight to make the right contacts, to present the manuscript in its best light, and to get the best deal. But the reality is, they can only do so much--the rest is in the editors' hands. They are the ones who, ultimately, make the decision to purchase or pass. Having a good agent increases your chances of a sale, but is no guarantee of anything. Agents often take on new writers, believing wholeheartedly in the project, only to be met with rejection from the houses. Sometimes it takes two or even three books before a client finally "sells." To what extent an agent may continue to push a project depends, I'm sure, on many factors--how much they love it, if there is a sure market for it, and if the feedback (even if mostly passes) has been positive.
I don't mean this to sound negative, just being realistic. But to reiterate, having an agent IS a major first step in the right direction. Good luck.
