Remember what Amalek did to you (25:17).
We must understand why there is still a mitzvah of remembering the act of Amalek today, even though in our present exile we would be forbidden to perform the mitzvah of eradicating him even if we knew for certain which peoples belong to Amalek.
In my humble opinion, the point of this mitzvah is to remind us now that it is possible for any creature of flesh and blood to become as wicked as Amalek, and like him to deny Hashem's role in the world even though he sees irrefutable evidence of it, as Amalek saw at the Splitting of the Sea and in the other miracles Hashem did for us in the Wilderness. The Sages (Midrash Tanchuma, Ki Seitzei 9) compared Amalek to someone who, seeing a bath of boiling water which all others were afraid to touch, nonetheless leaped into it. Despite the fact that he himself was badly scalded, he cooled the bath water to a degree where others could then also take the plunge of bathing in it. Similarly, all the miracles Hashem had done for the Jews did not deter Amalek from attacking them and making it possible for others also to want to wage war against them.
The lesson we learn is that each of us, however great his spiritual accomplishments, must worry that he himself might be tricked into committing the most serious sins, even those that everyone considers to be most despicable. Just as Amalek fell so low, we must also be afraid that any of us can fall equally low. Not only must each of us distrust his ability to persist in the good practices he has established for himself, he must also be continually on his guard for even the most serious sins, such as theft, murder, adultery, and the like.
Thus, after all the fervent prayers and confessions we offer on Yom Kippur, the Sages still required us to entreat Hashem in the Ne'ilah prayer on Yom Kippur to help us withdraw our hands from oppression. From this we see that no one may consider himself immune from committing even the gravest of sins, and allow himself to concentrate only on strengthening his positive aspects. No! All of us are made of the same flesh and blood as Amalek and the wicked of all other generations, and when the Sages warned us (Avos 2:5), Do not believe in yourself until the day of your death, they had each and every one of us in mind, and were afraid that anyone, without exception, could stumble in any of the mitzvos of the Torah.