As you think, so shall you be. Thus writes Swami Vivekananda in Karma Yoga: The Yoga of Action:

If good impressions prevail, the character becomes good; if bad, it becomes bad. If a man continuously hears bad words, thinks bad thoughts, does bad actions, his mind will be full of bad impressions; and they will influence his thought and work without his being conscious of the fact. In fact, these bad impressions are always working, and their resultant must be evil, and that man will be a bad man; he cannot help it. The sum total of these impressions in him will create the strong motive power for doing bad actions. He will be like a machine in the hands of his impressions, and they will force him to do evil. Similarly, if a man thinks good thoughts and does good works, the sum total of these impressions will be good; and they, in a similar manner, will force him to do good even in spite of himself. When a man has done so much good work and thought so many good thoughts that there is an irresistible tendency in him to do good in spite of himself and even if he wishes to do evil, his mind, as the sum total of his tendencies, will not allow him to do so; the tendencies will turn him back; he is completely under the influence of the good tendencies. When such is the case, a man’s good character is said to be established.

As the tortoise tucks its feet and head inside the shell, and you may kill it and break it in pieces, and yet it will not come out, even so the character of that man who has control over his motives and organs is unchangeably established. He controls his own inner forces, and nothing can draw them out against his will. By this continuous reflex of good thoughts, good impressions moving over the surface of the mind, the tendency for doing good becomes strong, and as the result we feel able to control … the sense-organs, the nerve-centers. Thus alone will character be established, then alone a man gets to truth. Such a man is safe for ever; he cannot do any evil.