Thomas Boston, Works, 4:222:
Sanctification by union with Christ through faith, 1 Cor. 1:2, Acts 26:18, is a mysterious way of sanctification unknown to the Jewish rabbis and Greek philosophers; an imaginary sanctification in the eyes of all legalists. The life of faith, emptying the man of himself, counting all his doings and sufferings loss and dung; doing every duty in borrowed strength, standing on borrowed legs, seeing with borrowed eyes, bearing burdens with borrowed strength; Christ being all to the man, and himself nothing, is a mysterious practice.
Source: http://www.puritanboard.com/f25/life-faith-mysterious-practice-84156/, Comment #1