Stephen A. Douglas and Slavery.

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. 

Economic 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 unambiguous.

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.