It is a default presumption amongst historical scholars that Lincoln's Illinois rival was at heart anti-slavery.
However, while conceding that it is a complex issue, this essay from 2005 makes a strong case that the Little Giant was actually moderately pro-slavery:
Consider what might be called the various
"types" or "categories" of antislavery. One could be antislavery in
the abstract, possessing what might be called antislavery
sentiment, for a wide variety of not necessarily congruent reasons.
The most prominent abolitionist argument was that slavery was a sin
against God, and conjoined to that indictment was the claim that
slavery brutalized black people.
In addition to these explicitly
moralistic abolitionist rationales, political critiques of slavery
also proved potent, such as the idea that slaveholders were a class
of aristocrats who leveraged their antidemocratic control of
southern society and politics to dominate the federal government at
the expense of northern free men. Many northerners, such as
Lincoln, also expressed alarm that slavery violated the nation's
creed of freedom and made a mockery out of national ideas.
Significantly, advocates of these political antislavery arguments
did not need to subscribe to northern evangelical tenets, consider
blacks equal, or even have sympathy for the plight of slaves. They
simply had to desire that the idea of freedom and the interests of
free states predominate in the national government.
concerns also generated antislavery sentiment, as illustrated by
the Republicans' free labor ideology. The Republicans castigated
slavery for limiting the opportunities of poor whites in the South,
curtailing the economic aspirations of nonslaveholders in the
western territories, and stunting the growth of the northern
economy. More generally, Republicans insisted that southern
opposition to high tariffs, free homesteads, and internal
improvements inhibited both individual mobility and its
concomitant, social progress. As with the moral and political
arguments, economic antislavery ideas possessed their own inner
logic and were not dependent on any other antislavery claim. Thus,
to take one example, many inveterate racists avidly supported
prohibitions on territorial slavery in order to preserve the
western territories exclusively for whites.
The application of these categories to
Lincoln and Douglas in the late 1850s is quite revealing. Although
some historians have claimed that not much separated the two
candidates on slavery, in fact Lincoln opposed slavery for all of
the reasons listed above excepting probably its sinfulness, while
Douglas opposed slavery, if he did at all, only due to its economic
consequences, and even on this subject his beliefs were not
While it is correct that Lincoln's religious views in 1858 were probably such that slavery-as-a-sin did not enter into them, the same could not be said as of the Second Inaugural Address. As with all things Lincoln and religion, the crucial question you have to ask is "When?"
I will add that, though beyond the scope of the above essay, it is beyond dispute that Douglas was definitely anti-black, and his race-baiting throughout the 1858 campaign was incessant and vicious.