From The Anand Sahib (A hymn attributed to Guru Amar Das)

 Knowledge of the Transcendent is not to be obtained,

Though outward religious observances, without knowledge,
Doubt and delusion will not depart;
No amount of outward observances
The mind is filthy with ignorance,
How can it be made clean?
Wash thy mind, O man, in the light of the Word,
And fix thy heart and thy soul upon the Lord.
Saith Nanak: It is by the grace of the Guru
That knowledge of the Transcendent is obtained.
In this way only will doubt and delusion depart.