The Unforgivable Sin
One of the few exceptions to the general rule that the penalty of lashes applies only to a violation of a Torah prohibition through action is the prohibition mentioned in the third of the Ten Commandments: "You shall not take the name of G-d, your G-d, in vain, for G-d will not absolve the one who takes His name in vain". (Shmot 20.7)
Rabbi Shimon bar Yochai deduces from the wording of this command that it is G-ds Heavenly court that does not absolve him of the sin of swearing falsely or needlessly, but allows the earthly court to absolve the violator of his sin by punishing him with lashes, even though he was guilty of speech rather than action.
The unique dimension of G-d not absolving the violator of this prohibition is brought to the attention of one who is about to take an oath to prove his claim in a monetary lawsuit. The judges warn him, say our Sages (Shavuot 39a), that in regard to all other sins the Torah says that one who repents will be absolved (Shmot 34:6), while in regard to swearing falsely such forgiveness is denied.
In Mesechta Yoma (86a) there is a dispute as to whether there is a difference in the nature of atonement required for swearing falsely and that required for any other transgression of a prohibition. Rambam (Hilchot Teshuva 1:2) rules that because of its unique nature, the sin of swearing falsely or needlessly is in the category of the serious sins which are not atoned for by the scapegoat on Yom Kippur without repentance as are all other transgressions which do not carry a penalty of death or extirpation. Two paragraphs later, however, when he writes that repentance combined with Yom Kippur achieves atonement for transgressions not punished by death or extirpation, he neglects to make the distinction between the sinful oath taken and other transgressors. (See Minchat Chinuch, Mitzvah 364.)
A Comitment of the Heart
What force is there to a commitment made in the heart but not verbalized?
In two places we find that such a commitment is binding. When Jews contributed materials for the building of the Mishkan sanctuary, the Torah describes these donors as "voluntarily giving of the heart" (Shmot 35:22). When the righteous King Chizkiyahu renewed the Beit Hamikdash service halted by his sinful father, the sacrifices brought by the people included "burnt offerings brought through the voluntary giving of the heart" (Divrei Hayamim II 29:31).
These two passages indicate that one who commits himself in his heart to voluntarily bring a sacrifice is obligated to do so even if he never verbalized his commitment. Since this rule is mentioned only in regard to these two matters of materials for the sanctuary and sacrifices, the gemara concludes that it is restricted to these areas, and one who takes an oath in his heart to do something or refrain from doing something is not bound to this commitment so long as he did not verbalize it.
What if someone today purchases property with the thought of dedicating it to the Sanctuary or makes a commitment in his heart to give charity? Is he bound to fulfill this unspoken pledge as in the above cases because of the similarity to them?
Two opinions on this subject are mentioned in Shulchan Aruch Choshen Mishpat (212:8) and Shulchan Aruch Yoreh Deah (258:13). In both places it is the ruling of Rema that it is proper to follow the more stringent opinion and fulfill the commitment made only in the heart and mind.