All activities minus God. This is material. And as soon as all activities plus God, it is spiritual. So all activities minus God means trouble, and all activities plus God is wholesome, is pleasing. So Kṛṣṇa consciousness means all activities plus God. That's all.
-Lecture -- Seattle, October 11, 1968

त्यक्त्वा स्वधर्मं चरणाम्बुजं हरे-
र्भजन्नपक्‍वोऽथ पतेत्ततो यदि ।
यत्र क्‍व वाभद्रमभूदमुष्य किं
को वार्थ आप्तोऽभजतां स्वधर्मत: ॥ १७ ॥
One who has forsaken his material occupations to engage in the devotional service of the Lord may sometimes fall down while in an immature stage, yet there is no danger of his being unsuccessful. On the other hand, a nondevotee, though fully engaged in occupational duties, does not gain anything.
-Shrimad Bhagvatam Canto 1 Chapter 5 Verse 17