Private, personal, secret prayer is a work of absolute necessity, says Thomas Brooks:
Private prayer is a work of absolute necessity, both to the bringing of the heart into a good frame, and to the keeping of the heart in a good frame. It is of absolute necessity, both for the discovery of sin, and for the preventing of sin, and for the purging away of sin. It is of absolute necessity, both for the discovery of grace, and for a full exercise of grace, and for an eminent increase of grace. It is of absolute necessity to arm us, both against inward and outward temptations, afflictions, and sufferings. It is of absolute necessity to fit us for all other duties and services.
For a man to glorify God, to save his own soul, and to further his own everlasting happiness, is a work of the greatest necessity. Now private prayer is such a work …
Thomas Brooks, The Secret Key to Heaven. Banner of Truth edition 2006, p97-98. First published 1665.