Why are Sifsei Sheker an abomination to Hashem?
R. Yonah: Above (17) it said that the Midah of a liar brings to false testimony. Now it adds that the Midah is To'avas Hashem; people who speak Sheker are one of the four groups that do not receive the Shechinah (Sotah 42a).
Mesilas Yesharim (Perek 11): This refers to people whose trade is Sheker. They concoct absolute lies in order to increase talk among people, or to be considered Chachamim who know many things.
Malbim: To'evah and Ratzon are opposites. Also Sifsei Sheker and Osei Emunah are opposites. One who promises to do something, whether a Mitzvah, Chesed or benefit to his colleague, if he does it, he spoke Emunah and he is faithful in his word. If he did not do it, he spoke Sheker and accustomed himself to promise and not fulfill. He is To'avas Hashem, because his speech does not match his deeds. Even though he spoke from the lips and outside (externally), he is a To'evah.
Why does it say "v'Osei Emunah Retzono"?
R. Yonah: It does not say 'Sifsei Emunah', for speech and thought of Emunah do not suffice, until he speaks with his mouth and fulfills with his hands.
Malbim: His deeds match his speech. This is Hashem's desire.
