The process of purification of mind, is like the refining process of gold. One needs to do away with the impurities (fetters/hindrances) to obtain pure gold. As long as they exist, one is confined to Samsara.

According to Theravada Buddhism, there are Three Gateways to Liberation. In the Visuddhimagga, they are described as realizing the Three Characteristics of Existence:

The Triple Gateway To Liberation

It enters upon the state of the triple gateway to liberation now with the predominance of [one of] three faculties according as the contemplation occurs in [one of] the three ways. For it is the three contemplations that are called the three gateways to liberation, according as it is said: “But these three gateways to liberation lead to the outlet from the world, [that is to say,]

(i) to the seeing of all formations as limited and circumscribed and to the entering of consciousness into the signless element,

(ii) to the stirring up of the mind with respect to all formations and to the entering of consciousness into the desireless element,

(iii) to the seeing of all things (dhamma) as alien and to the entering of consciousness into the voidness element. These three gateways to liberation lead to the outlet from the world”.

-- Visuddhimagga: The Path of Purification, p. 685-88, by Ven. Buddhaghosa