竊 (절) means "to steal" or "thief," but, interestingly, my Korean dictionary says it also means "마음속으로," which means "from one's heart." 以爲 (이위) means "to think," so if we combine "from my heart (竊)" and "[I] think (以爲)," we get "[I] believe." With that say, here is how I would translate the sentence:竊以爲捨本遂末終必無成“I believe that if we abandon the essentials to pursue what is peripheral in the end we will not succeed.”
The word 本末 (본말) means "means and ends," as in "a means to an end." In other words, this sentence seems to be saying, "Without a foundation, one cannot achieve success."竊以爲捨本遂末終必無成“[I] believe (竊以爲), [if we] abandon (捨) the means (本) to pursue (遂) the end (末), ultimately (終) [we] are sure (必) not (無) to succeed (成)."
By the way, my Korean dictionary gave the following example sentence: "본말을 전도하다." The sentence literally means, "Reverse the means and the ends," but my dictionary translated it as, "Put the cart before the horse."