I found an interesting articles that examines how 'undecided' voters think. This is not exactly about agnosticism but it relates to it in that a lot of agnostics tend to be undecided on various matters, as well. I'll post some excerpts from the article, mainly the parts that get into the psychology of undecideds. The following it's published by New America, a nonpartisan think tank.

In this report, we examine why a winning formula for undecided voters remains so elusive, and why we should be skeptical of anyone who thinks they’ve figured out the formula.
We define undecided voters as those who were not sure what party to vote for when asked who they would vote for if elections for the House of Representatives were held on the day they were interviewed.
Here, we present the full complexity of undecided voters, and their many uncertainties. In short, some agree with Democrats on some issues, Republicans on other issues; others agree with Democrats and Republicans on different sets of issues. There is no clear winning strategy, which may be one reason why no clear winning strategy has yet to be discovered.
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All types of voters have cross-pressures in their preferences, supporting some progressive and some conservative positions. But Democrats and Republicans have a higher share of voters who only support Democratic or Republican positions across these issues. About 26 percent of Democrats support only Democratic positions on social issues and about 15 percent of Republicans support only Republican positions.
They are largely disengaged from politics and hold beliefs that are idiosyncratic mixes of Democratic and Republican policies. Whether they are disengaged from politics because no party represents them well, or their beliefs do not conform to either party because they don’t pay enough attention to politics to hold anything close to a coherent belief system is difficult to say. But clearly, the two reinforce each other.
Unsurprisingly, undecided voters are on balance unenthusiastic and equivocal about both parties—they neither love nor hate either party. The lukewarm position of undecided voters likely reflects their inattention to politics or their lack of interest in learning more about parties and candidates, which given the polarizing forces of American politics would probably sort them into having very negative or very positive views of parties.
The lower interest in politics among undecided voters indicates that they might be more distant from political passions and have less intense feelings about politics compared to Democratic or Republican partisans. Their detachment from politics might suggest that undecided voters would have less well defined policy preferences, especially if their policy positions are not strongly informed by partisan cues or messaging. That they remain uninterested in politics even in districts where political messaging is more prevalent underscores their detachment from politics and the difficulties in creating meaningful linkages with them.
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If undecided voters do not feel particularly favorable or unfavorable toward either party, their vote choice is likely not driven by positive or negative partisanship. But, as we show in a later section, the policy preferences of undecided voters don’t seem to influence their views towards parties either, further compounding efforts at understanding this voting bloc. Even if they agree with the parties on key issues, this does not seem to push them closer to the Democratic or Republican party.
The only characteristics that seem to be consistent among undecided voters is that they are younger, less educated, less wealthy, and less politically aware and engaged than partisans—all factors that may contribute to their inscrutable inconsistencies, compared to more predictable partisans.
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I enjoyed reading the differences between the undecided voter versus partisan voters. Some of the differences are beneficial, some are not, some may not have any positive or negative effect at all.

Feel free to share your thoughts on undecideds and any details in the article.