To penetrate into the spiritual world, a thinking in accordance with truth is necessary. This requires  a clear recognition of how things stand. Thoughts are real entities, and untrue thoughts are evil, obstructing, destructive entities. […]

The ideal, which the man of today should set before his soul, is to be true; for thoughts are realities, and true thoughts are beneficial realities. Untrue thoughts — however well they may be enwrapped with the cloak of leniency as regards their own nature, — untrue thoughts received into man’s inner being, are realities which retard the world and humanity.

Source: Rudolf Steiner – GA 181 – Earthly Death and Cosmic Life: Lecture 4: The Cosmic Thoughts and our Dead – Berlin, 5th March, 1918

Translated by Harry Collison