A person may be great, wise, and full of the most excellent of virtues.
He may even be a zaddik and a Hassid.
But the essence of holiness comes to him only insofar as he is connected to God, the source of the holy.
A person may be connected with the source of holiness in several ways.
There is a holiness that is inherited, that belongs to the family, given by God to those who serve Him in a certain way.
Here one may include the holiness of Israel as a whole, or that of the sons of Aaron, the hereditary priesthood.
Then there is the more meaningful consecration that comes from the communion of man with God–such as may be attained, for example, through the mitzvot.
Adhering completely to the holy precepts for conduct and refraining entirely from wrongdoing envelops a man in a constant, ceaseless communion with God.
Beyond this is the more intellectual union with the divine holiness, through study and knowledge of the Torah.
When man puts his very life and soul into studying the Torah, and makes himself thoroughly familiar with the laws and the commandments, he becomes bound up in Torah, which is one of the manifestations of the Supreme Holiness.
Higher still is a man's ability to surrender himself, to relinquish his own will and being to God's will.
When a man reaches such a level of renunciation, he also attains a level of sanctification that reveals itself in different ways, according to his spiritual capacities.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose by Rabbi Adin Steinsaltz