There is discussion going on over at Travis’ as a result of the Barth Blog Conference. The question has arisen (of course it has) on whether Calvin and or Barth advocated for a Natural Theology (viz. that we can have a knowledge of God apart from God’s salvation grace in Christ). One of the protagonists over there (Shane, hi Shane 😉 ) is asserting that Calvin was in favor of a “Natural Theology;” which really means, that Calvin would be okay with Christ as “Saviour,” but not as “Revealor” of God (although he says this is not so, but I would like to see him explain that). In other words, that Jesus is not the source of our knowledge of God; but instead that He is the Saviour who merely “bridges the gap” (God of the gap theology) between our imperfect knowledge (i.e. “Natural Theology”) of God, and His “perfecting knowledge of God.” Here is what John Calvin says in his commentary on Colossians 1:15:
15. Who is the image of the invisible God. He mounts up higher in
discoursing as to the glory of Christ. He calls him the image of the
invisible God, meaning by this, that it is in him alone that God, who
is otherwise invisible, is manifested to us, in accordance with what is
said in John 1:18,
— No man hath ever seen God: the only begotten Son, who is in the
bosom of the Father, hath himself manifested him to us.
I am well aware in what manner the ancients were accustomed to explain
this; for having a contest to maintain with Arians, they insist upon
the equality of the Son with the Father, and his (homoousian) identity
of essence,  while in the mean time they make no mention of what
is the chief point — in what manner the Father makes himself known to
us in Christ. As to Chrysostom’s laying the whole stress of his defense
on the term image, by contending that the creature cannot be said to be
the image of the Creator, it is excessively weak; nay more, it is set
aside by Paul in 1 Corinthians 11:7, whose words are — The man is the
IMAGE and glory of God
That, therefore, we may not receive anything but what is solid, let us
take notice, that the term image is not made use of in reference to
essence, but has a reference to us; for Christ is called the image of
God on this ground — that he makes God in a manner visible to us. At
the same time, we gather also from this his (homoousia) identity of
essence, for Christ would not truly represent God, if he were not the
essential Word of God, inasmuch as the question here is not as to those
things which by communication are suitable also to creatures, but the
question is as to the perfect wisdom, goodness, righteousness, and
power of God, for the representing of which no creature were competent.
We shall have, therefore, in this term, a powerful weapon in opposition
to the Arians, but, notwithstanding, we must begin with that reference
 that I have mentioned; we must not insist upon the essence alone.
The sum is this — that God in himself, that is, in his naked majesty,
is invisible, and that not to the eyes of the body merely, but also to
the understandings of men, and that he is revealed to us in Christ
alone, that we may behold him as in a mirror. For in Christ he shews us
his righteousness, goodness, wisdom, power, in short, his entire self.
We must, therefore, beware of seeking him elsewhere, for everything
that would set itself off as a representation of God, apart from
Christ, will be an idol.
The emboldened part is the part I want to emphasize. Calvin did not hold to a “Natural Theology,” unless of course we construe it in ways that are consonant with his commentary here. That is, Calvin believed that knowledge of God apart from Christ only can end in idolatry (worshipping the creation); it is only as we “know God as ‘Redeemer'”, in Christ, that we can viably construct a “Natural Theology.” We need new “spectacles” (Calvin’s language); without them we can only worship creation.
Did Calvin hold to a “Natural Theology?” I don’t think so, not anymore than Barth did.
On a sidenote, I think T. F. Torrance is helpful here, esp. with his discussion on The Mediation of Christ. Torrance speaks of an “Order of Knowing,” which obejectively comes from Christ for us, in the incarnation. So that we are objectively brought into the life of God, prior to our ability to subjectively “know” God by the Holy Spirit through Christ. Obviously TFT is not in the “Natural Theology” camp, nor am I, per se.
P. S. Salvation isn’t an attribute of Jesus, Jesus (in the “economy” of God’s life) is Salvation! To hold to the former is to hold to the view that allows for “Natural Theology;” to hold to the latter, is to hold to the view that scripture endorses 😉 .