In his Reformed Dogmatics, Heinrich Heppe collects statements on what was lost to humans of the image of God as a result of the fall, and what still lingers:
All later Reformed dogmaticians adhere to the distinction established in Melanchthonian theology between the substance and the virtutes of the divine image, and they accordingly teach that the original virtus and the imago has been irretrievably lost.
Cf. Polan (VI, 5): “From this it is plain what of the divine image is left in corrupt man. There remained the substance of the soul; there remained the essential faculties of the soul, knowledge and will; there remained the essential attributes, as a kind of natural knowledge, reasoning power, judgement and thought, freedom from compulsion in the will; there remained natural life and the immortality of the soul. Therefore the image of the nature is not utterly destroyed by the sin of Adam and Eve; this must be credited to the mercy of God towards the human race. Nevertheless it has been lost in part and what is left is wretchedly corrupt and misshapen. Moreover all the rightness, i.e., the sanity and integrity of the perception and recognition of God and divine things, the original righteousness and holiness by which in particular man was a partner in the divine nature, has been completely destroyed, extinguished and left out”.
Hence this point of doctrine also belong to those who represented the difference between the Reformed and the Lutheran systems.
So, e.g., Wendelin (Systema, p.508): “There remains in man corrupted by Adam’s lapse a rational soul, which is an immortal spirit; there remain the faculties, thought and will; in thought there remain as though inborn the theoretical and practical principles of truth.�In short there is still some portion of dominion. Meanwhile none of these has been so acquired that by it fallen and corrupt man is able either to rise again or to prepare himself to receive the offer of grace, or to co-operate with God even when He is laying the first foundations of grace”.
Similarly in the Collatio, p.125: “We assert that the principal part of the divine image, namely original righteousness, was plainly lost and abolished through the fall and sin of origin. Meanwhile we deny that the entire image of God in all its parts was utterly lost and abolished, which those will easily concede who recognise part of the divine image in the rational soul as an immortal spirit endowed with thought and will. By the fall man did not cease to be man, although he did cease to be righteous”.
(“The Violation of the Covenant of Works”, slightly edited)