Descartes wants to "establish" some "firm and lasting" knowledge. (P 144a)

To do this he will "begin again from the first foundations." (P 144a)

First he will "raze everything" (P 144a), i.e. will engage in "the general destruction of my former opinions." (P 144b)

He need not show these opinions to be false (P 144b), because ...

"I should withhold my assent no less carefully from things which are not plainly certain and indubitable than I would from what is patently false ..." (P 144b)

I.e., if you can't be sure of it, don't believe it.

If there is a "reason for doubting" (P 144b) a belief, then one can't be sure of it.

And if one can't be sure of "the foundations" (P 145a), one can't be sure of beliefs that have been "built upon them" (P 145a)

So all Descartes needs to do (for this destructive project) is to show that there is reason for doubting the foundational beliefs.