Jonathan Edwards writes,
When God sets his seal on a man’s heart by his Spirit, there is some holy stamp, some image impressed, and left upon the heart by the Spirit, as by the seal upon the wax. And this holy stamp, or impressed image, exhibiting clear evidence to the conscience that the subject of it is the child of God, is the very thing which in Scripture is called the seal of the Spirit, and the witness or evidence of the Spirit. And this mark stamped by the Spirit on God’s children is his own image. That is the evidence by which they are known to be God’s children; they have the image of their Father stamped upon their hearts by the Spirit of adoption.
***Taken from Elliot Ritzema and Elizabeth Vince, eds., 300 Quotations for Preachers from the Puritans, Pastorum Series (Bellingham, WA: Lexham Press, 2013).