Action is the fruit of knowledge, and knowledge without any action is like a tree without any fruits.

'Abdullah Ibn Mas'ood رضي الله عنه said, "We would learn ten verses directly from the mouth of the Prophet, and we would not exceed that until we had learnt their meaning and acted upon them. We learnt knowledge and actions (together)."

Knowledge along with sound actions that have met the stipulated conditions (i.e. solely doing the act for Allaah سبحانه و تعالى alone and solely following the Messenger of Allaah صلى الله عليه وسلم), will be a proof for the slave with Allaah سبحانه و تعالى.

If knowledge and righteous actions, that have fulfilled the two conditions mentioned (above), are present together, then this knowledge will be a proof for the slave with his Lord. However, if there is knowledge and no action, then this knowledge will be a proof against the slave and this kind of person resembles those whom Allaah سبحانه و تعالى is angry with, and they are the Jews. They are categorised as, 'Those whom Allaah is angry with', because they failed to act upon their knowledge.

If there are actions without any knowledge, thus worshipping whilst ignorant and confused, then this kind of person has resembled the Christians, because they used to worship Allaah سبحانه و تعالى upon ignorance and misguidance.

Sufyan Ibn 'Uyainah رحمه الله said: "They used to say: whoever is corrupt from amongst our scholars has a resemblance of the Jews, and whoever is corrupt amongst our worshippers has a resemblance of the Christians." [Majmoo' Fatawa Ibn Taymiyya, 1/197]

It is mentioned in an authentic hadeeth: "A man will be brought on the Day of Resurrection and will be cast into Hell, and his intestines will pour forth and he will go round them as a donkey goes round a millstone. The inmates of Hell will gather round him and say: 'What has happened to you, O so-and-so? Were you not enjoining us to do good and forbidding us to do evil?' He will reply: 'I was enjoining you to do good, but was not doing it myself; and I was forbidding you to do evil, but was doing it myself." [al-Bukhari and Muslim]

In the past there was a famous saying that was often quoted: "A scholar who doesn't act upon his knowledge will be punished before the idol worshippers." [The Three Fundamentals of Islam]

See:

Virtues of Knowledge which is Learnt and Taught for the sake of Allah - http://www.muftisays.com/forums/du-aas-supplications/5075/virtues-of-knowledge-which-is-learnt-and-taught-for-the-sake-of-allah.html