Individual salvation based on the sincerity of the believer puts the realm of religion only in the heart or soul or mind of the individual. So, emphasis on individual sincerity (initially and eventually; first and finally) makes every religion equal to every other religion, for it is based on the sincerity/faith/belief of the person who seeks (particular) religious truth. The revealed knowledge of God is replaced with human cravings. Religion becomes humanistic, not divine. That is, how ardently a fellow "means" his religion is more important than what his religion "means" apart from him.
Sincerity, historically and politically, trends to the left. Cynicism/skepticism actually trend (historically, politically) to the right. Sincere individualism actually holds that larger bodies, which tend always to self-correct to the left (until they don't any more), can be trusted to do the greater good for the larger number. In fact, larger bodies (nations, armies, religious bodies, et al) or systems that self-correct quickly actually favor (by favor, I mean act according to the stated wishes of) a smaller agenda driven body within their perceived constituency. The smaller constituencies may actually emanate from the core of the larger body but relate to the larger body only marginally.
So, the Abolitionists steer the helm of Republicanism, though Abolitionism is only a small part of the Progressives, Copperheads and Know Nothing groups emergent from the old American Whig party. The Abolitionists succeeded in correcting their greater group quickly to the left because they seized on the Original Sin of the American system.
The cost was 600,000 plus American lives.
To score religion as primarily individual in nature denies the New Testament portrayal of Jesus' teachings. Individualism if isolationist, so a Christianity set in individual salvationism trades the Gospel of Luke for On Walden's Pond. Christianity is social, social in a gritty, get in your grill kind of way. From the beginning of His ministry, Jesus calls disciples and speaks to crowds. Yes, he does remove Himself from some gatherings but his recusals matter because they are exceptional, not normative.
Opinions expressed here are mine alone.