In the broadest terms, I don't think that it is either an advantage or a disadvantage.

There is no such thing as true objectivity; we are all products of our time, and we all have our biases that make absolute neutrality impossible. Historians are not perfect, and we cannot flip a switch that lets us change from real, usually quite ordinary men and women into perfect adjudicators of the past. We have religious beliefs, political opinions and social views just like any other member of the society in which we live. Very few people are truly 'neutral' about the big questions in life and society; what is often regarded as neutrality is often perhaps better described as centre-groundism, and is in and of itself a kind of 'political' position. In the case of your question, an individual who considers themselves truly religiously agnostic is not going to be objective - a declaration of uncertainty in belief reflects a failure to be convinced by either position, which is in and of itself a statement on both religious belief and non-belief.

The great thing about the historical method - the set of tools and skills we use to write history - is that it is a methodology that imposes extremely high standards for evidence. Historians are required to construct their arguments from a wide range of evidence. That means that they cannot cherry pick material from the historical record that suits them best; good works of history are able to take seemingly contradictory bodies of source material and reconcile them together, synthesising from the evidence we have a coherent and compelling argument. Historians critically examine one another's work and explore different possible interpretations of the historical record in a bid to try and reach a consensus on a narrative of history that seems to best match available evidence. Though it is impossible to ever entirely eliminate bias, this is a process that serves to greatly mitigate the risk of it having an overt impact on historical work, not in the least because it invites scholars to anticipate the potential criticisms of their research and resolve (or have rational justifications for) flaws in their work in advance.

Historians likewise strive to be mindful that our modern experiences and understandings are not particularly useful for understanding of the past; people have always been people, yes, but that means they have always been infinitely complex. Christian experience in the 21st century is not very useful for understanding Christian experience in the 17th century; Christian experience in the 17th century is not going to be useful for understanding Christian experience in the 5th century. There is no particular reason why someone who believes in a higher power today should have an advantage in understanding religious belief in historic contexts or other cultures; historians try to understand the people of the past as they understood themselves where possible, and earnestly try to not project modern ideas onto historical contexts. Whilst modern experiences and beliefs can and do arouse empathy and sympathy with people of the past, those feelings should not be mistaken for authentic understanding. I am naturally enormously sympathetic to the plight of enslaved African Americans in the antebellum South, and not very personally sympathetic at all to those who enslaved them - but as a historian of slavery, it is important that I try as much as possible to understand both groups of people on their own terms, in their own words and ideas, and in their own time. My personal and emotional response to the historical record is not necessarily my intellectual response.

Religious belief is certainly going to influence the emotional response an historian has to the past, and it may well drive their research interests - it is common for people to pursue the history of groups they identify with. Hopefully though, the demands of the historical method will serve to separate the emotional response an historian personally experiences from the intellectual response they exhibit in their work. Just as it is possible for an historian to strive to understand someone they abhor morally and have no sympathy with, so too is it possible for someone to set aside their personal sympathies and critically consider an historical event or figure. The demand for making arguments from critical consideration of primary source material - that your argument must follow the evidence, not the other way around - is a powerful check on those who would try to advance a personal agenda through writing history. Any good scholar is going to want to strive to understand all people and events as earnestly as they can. That is true even where it is largely acceptable to be particularly in favour of one group in the historical record. If I want to understand the experience of slavery for example, then I need to earnestly understand the people responsible for that atrocity just as much as those subjected to it.

For my own part, I am someone who today has come to hold quite strong religious convictions, though I didn't when I began my historical training. I think perhaps the most significant impact that has on my own approach to history is that it keeps me mindful that, from my own perspective as someone who believes in the eternal life of the soul, the people that I study are not really 'gone'. They're still very much real - people who I could very much one day meet in some way myself - and I would like to think that, in whatever form the after life takes, the work of historians to recover the record has been appreciated. As someone who likewise believes in universal salvation, that applies just as much to those individuals whose actions I find to be morally abhorrent. But these are emotional responses to the past that motivate the way I personally approach my work; when I was an atheist, I would have argued that precisely because there is no after life, earnesty in historical research is so important because the people of the past had only their lives and experiences, and so those experiences deserve to be understood as authentically as possible both as memorial and lesson. I don't think there is an inherent advantage to either philosophy I have subscribed to in my lifetime - both lead me to value the same methodological approaches and conceptual frameworks for studying the past.

Now, I'm obviously not someone who works on religious history directly - though religion is an enormously important part of the history of New World slavery - and those who do may have differing views with regards to the specific quirks and characteristics of their disciplines. For historians broadly though, the same principle applies throughout. Religious belief or lack thereof is just one of many different biases that historians have to carefully account for in their work, and there is no broad inherent value in being on one side or the other of that fence. Though religious belief and non-belief are inevitably going to shape your moral and emotional responses to the people and events of the past, there is no reason why they must or should distort your methodological approach, and it is the carefully developed skills, tools and principles of the historical method that - properly applied - serve to ensure works of historical writing are not unduly biased. Bias can never be fully removed, but it can be mitigated.