Principles of Faith

R. Hayim Soloveitchik's strong stance on the denial of a principle of faith is well known through the writings of R. Elhanan Wasserman. The following is a fascinating confirmation that I found in Making of a Godol (p. 538 - emphasis added):

The closest that R' Hayyim came to coercion according to Toras Hayim was the explanation that he gave of Eliyahu haNavi's suggestion to the idol worshippers who were "hobbling between two opinions", to wit, "If G-d be the L-rd, follow Him; and if Ba'al, follow him." R' Hayyim asked how the Prophet could possibly suggest they worship Ba'al, and he answered that in principles of faith, the denial of even one tenet makes one's acceptance of all the rest worthless. Therefore, if Eliyahu's listeners had a slight tendency to Ba'al, their worship of G-d became useless anyhow, and he suggested to them that they may as well go all the way to Ba'al. He added that the 850 prophets of Ba'al and the Asherah were "certainly not ministers or priests of the Ba'al church, but rabbis who ruled on every mitzvah properly, except that they incorporated the worship of Ba'al into the Torah" - and that nullified all the good deeds they were preaching and promoting.

Note that he did not say that the good deeds of a sinner are worthless. The Rambam in his Iggeres Teiman decried such an attitude. R. Hayim was only speaking of one who denies a principle of faith.