Just as blazing fire reduces all the firewood into ash, the fire of Knowledge burns off all karmas into mere ash.
Activity is generally intended to gain objective outcomes, like walking to reach a destination, eating to appease hunger and bathing to clean the body. Long-term pursuits like studies, lasting for decades, are also there. Any act aims at its fruition. Spirituality does not refer to objective outcomes, ingrained in activities themselves.
Subjective results like sin and virtue, good and bad, favourable and unfavourable run in parallel. They fall under desirable, undesirable and their mixture.
Krishna’s statement is linked to Arjuna’s primary confession: “We are out for a colossal sin.” (1.45) Krishna assures no such sin would be, as the fire of wisdom burns away all mental effects of actions. What a great relief, assurance and hope!
Spiritual knowledge is like light on darkness. Even long- prevailing darkness of a cave flees when a lamp is lit there. Ignorance makes one think of sin. It vanishes in front of wisdom.