Vishnu sahasranamam, slok53

Slok53         Uttaro  GOpathirgoptha   Gnaanagamyaha  PurAthanaha
                      Sareera bhoothabhruth  Bhoktha  KapeenDro Bhuri DakshiNaha

The  North, the lord of cows, the celestial Secret,( invisible),  the Ultimate Destination of true knowledge, the Ancient, the Driving force in living bodies,  the enjoyer of the fruits of actions,  the Hanuman, the South of the earth, is Sree Maha Vishnu. 

Vishnu is the north and south of earth.
Vishnu  is Sree Krishna, and also Sree Hanuma who is the  symbol of lord Siva.
Vishnu  is the "Chaitanyam", the vibrancy in all living  beings.
Vishnu  is not visible to impure souls.. The " Absolute Purity" is visible only to pure souls.
It is" GnAnam" means true knowledge which alone can change an impure soul to a pure soul.

Knowledge of truth acts like a detergent, cleanses all bad karmic currents of a soul.

Hence, Vishnu stands as the purifier and the goal of purity.

Sree krishna is one incarnation of  Lord Vishnu, in Dwaparayug. Krishna  is Vishnu.
 Here, in this slok, the Dwaperyug, ThrethAyug prime manifestations, are emphasised.

Om Vishnave namaha