Please read the following important quotes from Ch'an Master Sheng Yen. They tell you why it is necessary to take refuge in the Three Jewels.
http://www.dharmadrum.org/content/chan_garden/chan_garden2.aspx?sn=40
Why is it important to Take Refuge in the Three Jewels?
... Participating in the ceremony of taking refuge in the Three Jewels is the first, important step for anyone who wants to become a Buddhist. Why? Because the heart of Buddhism is the Three Jewels of the Buddha, Dharma, and Sangha...
... [Taking refuge] is about committing one's life towards a path to awakening, which is, in fact, freeing not binding ... If we realize that the Buddhist teaching is beneficial or meaningful in our lives, then the next step is to take refuge in the Three Jewels. When we become Buddhists, we commit ourselves to bringing genuine liberation to ourselves and to everyone around us. This is the Buddhist path ... Trying to learn Buddhism without taking refuge is to be a bystander and not a participant...
... Those who believe that having a pure, sincere heart is enough to qualify them as Buddhist practitioners and who see no need to go through the formal refuge ceremony, are not really Buddhists. If you want to get an education, you must first register and then proceed through elementary, middle, and high school until you reach college—perhaps reaching as far as a Ph.D. It is impossible to progress in one's education without taking these successive steps.
Similarly, self-proclaimed Buddhists are not real Buddhists. They are like people who are fond of another country, emigrate there, pretend to be citizens, but never apply for citizenship. Those who refrain from taking refuge, but insist upon calling themselves Buddhists, may glean some benefit from the teachings, but the essence of Buddhism will always elude him. Taking refuge is a required process, not an option. The sutras or Buddhist scriptures tell us that even people who perform good deeds will not be able to eradicate bad karma unless they take refuge in the Three Jewels...
... Some people believe that their comprehension of the Buddhist sutras, which they take to be one and the same as the Dharma, is sufficient to enable them to advance directly to full enlightenment. They see no need to practice meditation or receive the Three Refuges. While this may have its appeal, it is a serious mistake.
The Buddhist sutras were taught by the Buddha and his disciples, and later collected and written down by members of the Sangha. Concentrating on these texts only yields a limited understanding of the Dharma Jewel. This would lead us to disregard the Buddha, who gave these teachings, and the Sangha, who spread the Dharma. Buddhism stresses the Dharma—the path which leads to the ending of suffering—only in conjunction with the Buddha and the Sangha. The three are inseparable. It is true that taking refuge requires investigation of the Buddha's teachings, but it also necessitates participation in the refuge ceremony, which must be conducted by a precept master, who is usually a member of the Sangha. This confers the formal recognition that you are a Buddhist...
... Lastly, I must stress that taking refuge in the Three Jewels is only the first step in becoming a Buddhist. Once we have taken refuge, we must embark on the path of spiritual cultivation, which involves finding a teacher and developing a regular practice. Doing so will decisively set you on the Buddhist path of awakening to wisdom and compassion...
" Just as a sick person relies on a wise doctor to prescribe medicine and on nurses to help them, I as a person suffering from the constantly recurring ills of cyclic existence, now turn to the Buddha, a skillful doctor who prescribes the medicine of the Dharma--ethics, concentration, wisdom, altruism, ... The Sangha act as nurses who encourage me and show me how to take the medicine. However, being surrounded by the best doctor, medicine and nurses will not cure the illness; the patient must actually follow the doctor's advice and take the medicine. Similarly, I need to follow the Buddha's guidelines and put the teachings into practice as best as I can. "
~ Ven. Thubten Chodron