Deuteronomy 11:27 – 28.The blessing, that you will listen to the commandments of the Lord your G-d, which I command you today; and the curse, if you will not listen to commandments of the Lord your G-d, and you will turn away from what I command you this day, to follow other gods, which you did not know.
Sifri: From here they say that anyone who accepts idol worship is as though he denies the whole Torah; also, anyone who accepts the Torah is as though he denies idol worship
Torah Temimah – Colloquial translation of Note #66 (second paragraph)
In general we need to examine the issue of why the blessing seems to be short and abbreviated – just saying that you will listen. The curse, on the other hand, explains in more detail saying, “You will not listen and you will turn away….”
Perhaps the explanation is according to what it says in the Gemora Kedoshim 40(a) and in other places. There it says that a good thought is considered by G-d as though you have also done a good deed; however, G-d does not do the same with a bad thought.
Therefore, with the blessing it is sufficient to say merely “you will hear”. [That is the blessing, in and of itself!] In other words, you will hear and accept these words. If you do this, you will immediately merit the reward.
On the other hand, for a curse, you will not merit a curse just by not listening. Rather, you must also do an actual bad deed, “turn away from what I command you this day”. Therefore, as long as you don’t actually do a bad deed, your bad thoughts do not bring you to a punishment.
Translator Note: I think that this is an important point. We can’t go around beating ourselves up over bad thoughts. They don’t “count” against us unless we put them into action.