Rabbi Mayer Twersky
Teshuva: In Your Mouth and In Your Heart
"ki hamitzvah hazos asher Anochi metzavcha hayom lo niflais he mimcha vlo rechoka hi...ki karov eilecha hadavar meod bificha vblevavecha laasoso - for the commandment that I command you today is not hidden from you and it is not distant...Rather the matter is very near to you - in your mouth and your hear - to perform it." (Devarim 30:11,14 Artscroll Stone Edition translation)
According to the Ramban, the mitzvah depicted in these verses is the mitzvah of teshuva (repentance). The Torah emphasizes our capacity and ability to repent. It is "in your mouth and your heart" to repent.
Let us focus on the Torah's choice of words - "in your mouth and your heart." Clearly, the Torah is conveying that teshuva is very doable, but that has already been conveyed by the first half of the verse, "[it] is very near to you". What is added by the phrase "in your mouth and your heart"?
This terse, rich phrase anticipates the myriad excuses that we offer for our failures to do teshuva. Our first line of defense is that we do not need to do teshuva. We are not at fault. After all, we are only human. And to be human is to sin. Alternatively, we silence our consciences by reasoning that our sins are not our fault. Our sins are due to our upbringing, society, genetics, etc. In a word, we do not assume responsibility for our sins. The Torah utterly rejects such moral escapism. "Free will is bestowed on every human being...the human species had become unique in the world...there is none who can prevent him from doing that which is good or that which is evil" (Rambam Hilchos Teshuva 5:1). Being human is not a source of extenuation, but rather moral responsibility. "Thus Yirmiyahu [Jeremiah] said 'out of the mouth of the Most High not evil and good'; that is to say, the Creator does not decree either that a man shall be good or that he shall be wicked" (ibid, halacha 2).
The primordial ploy of shifting the blame - "The woman whom you gave to be with me - she gave me of the tree, and I ate" "The serpent deceived me, and I ate" (Breishis 3:12,13 Artscroll translation) - was rejected by Hakadosh Baruch Hu in time immemorial. The modern equivalents - my upbringing is responsible, society is to blame, there is no overcoming genetic tendencies - will share the same fate.
Thus the Torah exhorts us that teshuva is "in your mouth...to perform." The teshuva process begins "in your mouth", by confessing and thereby accepting responsibility for our sins.
When our first line of defense for not doing teshuva fails, we seek other justifications for out inaction. One common excuse appeals to age. "I am too old; my habits are too deeply entrenched. After all, you can not teach an old dog new tricks." What is the Torah's response to this hackneyed excuse? "For You do not wish the death of one deserving death, but that he repent from his way and live. Until the day of his death You wait for him; if he repents You will accept him immediately." [Mussaf, Yomim Noraim, Artscroll translation]
At times, we attribute our failures to repent to the magnitude of our sins. "I have sinned too egregiously; I am too mired in sin. How can you expect me to do teshuva?" The Rambam (Hilchos Teshuva 2:1) paraphrases the Torah's response, "Even if he transgressed throughout his life but repented on the day of his death and died as a penitent all his sins are forgiven." Even a lifelong sinner can, and therefore must, repent.
And, finally, another favorite excuse id "if only someone would help me. If only my Rebbeim z"l were still alive..." The Gemara (Avodah Zarah 17a) debunks this excuse by depicting the teshuva of Rabbe Elazar ben Durdia. Rabbe Elazar had led a life of wanton promiscuity; he had consorted with every harlot in the world. Finally impelled to seek forgiveness he petitioned the mountains and hills to implore on his behalf. They declined, citing their need to pray on their own behalf. Next he addressed himself to heaven and earth, seeking their intervention on his behalf. Their response was identical to that of the mountains and hills. Then Rabbe Elazar appealed to the sun and moon with the same negative results. Finally, he said, "It [i.e. attaining forgiveness] in entirely dependent upon me. He rested his head between his knees and burst out crying until his soul departed. A heavenly voice emerged and proclaimed, 'Rabbe Elazar ben Durdia is prepared and deserving of the world to come'". The "if only" excuse is just that, a shallow, hollow excuse. Ain hadavar taloi ela banu; repentance depends entirely upon us.
The Torah rejects our second line of defense - "I am only human", "I am too old", "if only..." Teshuva "is in your heart to perform". If only we inwardly resolve and strive, we can, with Hakadosh Baruch Hu's help, repent, v'chain yehi ratzon.
 Ramban, ad locum, provides a pshat interpretation that "in your mouth" refers to viduy (confession) and "in your heart" adds that, the indispensability of viduy notwithstanding, the essence of teshuva is an inner experience, a kiyum shebalev. What ensues is a homiletical interpretation.