Those who could not learn [the] simple lesson in karma could not hope to comprehend the law of dependent origination or the Four Noble Truths. Comprehension of the triple doctrine enabled aspirants to cleanse their minds of false doctrines and preconceptions. Once the dross had been purged away, they were ready to move on to more advanced teachings. In other words, the Buddha employed a method of gradual guidance leading from the easy to the hard. Several primitive sutras contain passages like the following, explaining this gradual approach:

“To one seated before him, the World-honored One preached the Law gradually. First he taught giving, obeying the precepts, and [thereby] rising to heaven. Then he explained that selfish desires are evil and the cause of both misfortune and impurity, whereas separating oneself from selfish desires is a great virtue. This teaching made the heart of the listener malleable and receptive; the person put aside prejudices and experienced the ecstasy of faith. When this happened, the World-honored One for the first time preached the Four Truths of suffering, desire, nirvana, and the Way. As freshly washed and bleached fabric receives the dye, the listener received the Four Truths. While still seated there, the listener acquired the pure and unsullied Eye of the Law [which sees that all things arising from causes are ultimately extinguished].”
Basic Buddhist Concepts