Once spiritually born of God, we have received His nature. This means that we are already perfect in the composition or quality of the Spirit. Therefore, we cannot further improve in our spiritual nature. However, we «are renewed after the image of him that created us,» i.e., regularly restore to His original, perfect image.
For which cause we faint not; but though our outward man perish, yet the inward man is renewed day by day. (2 Corinthians 4:16)