The Gemara (Avodah Zarah 3.) states that in the future, the goyim will come to Hakadosh Baruch Hu and ask, "Why do you give reward solely for Bnei Yisrael? How do you know that they kept the Torah?" Hashem replies, "Heaven and earth will testify that the Yidden kept the Torah." The goyim say, "Heaven and earth aren't valid witnesses because they are בעדותן נוגעין; they have a personal bias in claiming that the Yidden keep the Torah. This is because Hashem made a condition with heaven and earth that if the Yidden don't keep the Torah, heaven and earth will be destroyed. They certainly are interested in saying that the Jewish nation kept the Torah, and their testimony is rejected.
Tosfos asks, isn’t the existence of heaven and earth sufficient proof that the Yidden keep the Torah? Even without their testimony, the fact that heaven and earth exist proves that the Yidden keep the Torah. Otherwise, heaven and earth would have turned into nothingness.
Tosfos replies, "Heaven and earth exist because Bnei Yisrael accepted the Torah [at Har Sinai]." The condition was that Bnei Yisrael should accept the Torah at Har Sinai, and they did. Even if they don't keep the Torah later, heaven and earth continue to exist in all generations due to their kabbalah in the past.
For a moment of a kabbalah tovah (good resolution), when the Yidden received the Torah, the entire world stands.
People may ask, "Why should I make a kabbalah tovah? I know that sooner or later, I will return to my old ways." But this shouldn’t be a deterrent. Remember that heaven and earth stand forever for one moment of a kabbalah tovah. The same goes for you. The world will stand in your merit because of your sincere kabbalah tovah, regardless of whether you ultimately keep it.
Furthermore, if you make a kabbalah tovah, you will likely keep it. As Chazal say, אותו מסעיין לטהר הבא, when a person desires to be pure, he is helped. Heaven will help you maintain your kabbalah tovah.
It states (Shemos 15:26) לְקוֹל ַעְׁמִּשת ַָמוֹעׁש ִםא ֶרוַיֹּאמ אלקיך 'ה , the Chasam Sofer explains, "I thinks it is a guarantee, ַָמוֹעׁש ִםא if you make a kabbalah tovah to listen and to obey, I guarantee you that אלקיך 'ה לְקוֹל ַעְׁמִּשת, you will listen and keep Hashem's command. Hashem will help you..."
It is known that "all beginnings are hard" (see Rashi 19:5). It is difficult to change one's habits and do ratzon Hashem, which makes avodas Hashem so hard. But after one becomes accustomed to new customs and ways, it isn't hard anymore, and avodas Hashem becomes sweet and easy. Rashi writes this on the pasuk (19:5) ַָמוֹעׁש ִםא ָּהַתוְע בְּקֹלִי ְ עוְּׁמִּשת , "And now, if you will listen and pay attention to My voice..." Rashi writes, "If you accept it now, the Torah will eventually become sweet for you because all beginnings are hard."
Rebbe Yehoshua of Belz zt'l teaches that when the "beginning" is a kabbalah tovah, the beginning is easy too. This is because the yetzer hara doesn't know that the person made a kabbalah tovah. The kabbalah tovah is in his mind, and the yetzer hara – a malach – cannot read people's thoughts. So, his first step, taking upon himself the kabbalah tovah, was accomplished without opposition from the yetzer hara, making avodas Hashem easy for him.
This is hinted in Rashi's words, עתה אם עליכם תקבלו , if you will make a kabbalah tovah, לכם יערב ואילך מכאן, from now on avodas Hashem will be sweet and easy for you.
