Before the initiation, you have to ask the Guru whether the Guru will be willing to give a Shiva mantra or not. Paru says, will Guru give the mantra Dixa as well suggest Istadheva to meditate upon? At that level, this entire universe is one reality. Everybody is one with you or you are one with everybody and everything. So that is that oneness. You can let go of all thinking at that level.
Swami Sarvapriyananda teaches the Katha Upanishad, one of India's most revered texts which is also a part of the Vedas. It is a collection of philosophical poems representing a conversation between the sage Naciketas and Yama (god of death). They discuss the nature of Atman, Brahman and Moksha(liberation).