Thomas Goodwin, Christ Set Forth:
There are assuring promises; as those made to such and such qualifications of sanctification, etc. But still what is it that is promised in them which the heart should only eye? It is Christ, in whom the soul rests and hath comfort in, and not in its grace; so that the sight of a man’s grace is but a back-door to let faith in at, to converse with Christ, whom the soul loves. Even as at the sacrament, the elements of bread and wine are but outward signs to bring Christ and the heart together, and then faith lets the outward elements go, and closeth, and treats immediately with Christ, unto whom these let the soul in; so grace is a sign inward, and whilst men make use of it only as of a bare sign to let them in unto Christ, and their rejoicing is not in it, but in Christ, their confidence being pitched upon him, and not upon their grace; whilst men take this course, there is and will be no danger at all in making such use of signs. And I see not, but that God might as well appoint his own work of the new creation within, to be as a sign and help to communion with Christ by faith, as he did those outward elements, the works of his first creation; especially, seeing in nature the effect is a sign of the cause. Neither is it more derogatory to free grace, or to Christ’s honour, for God to make such effects signs of our union with him, than it was to make outward signs of his presence.
Source: http://www.puritanboard.com/showthread.php/87980-A-back-door-to-let-faith-in, Comment #1