Anyway, to the point. Morwenna Ludlow has written a book about books that people have written about the works of Gregory of Nyssa, one of the Church Fathers (e.g. old dead guy, theologically influential) who was involved with formulating the doctrine of the Trinity, and in it she looks at the question of what affects the way we read particular texts. She concludes that one of the key things that affects the way someone reads the Fathers is their understanding of the overall shape of Christian history, and she identifies three main models:
1) The 'static' model of Christian history thinks that theology and the Church are basically unchanging, and insofar as Christian doctrine has developed, it's just the working out of the logical implications of the original message: so you won't find the doctrine of the Trinity as such in the New Testament, but if you read it right it's obviously implied, it just took people a while to figure it out. People who have this view tend to think that one particular Christian tradition (e.g. Roman Catholicism, Eastern Orthodoxy) is more right than all the others, and that the role of theology is just to keep people sticking to the party line, the original gospel. They tend not to like the idea that anyone might introduce 'new' ideas into the Church.
2) The 'reformatory' model agrees with the static model that Jesus' original message was wicked cool and complete, but thinks that somewhere along the way, the Church screwed it up and the message got lost, so now we need to 'recover' the original gospel message. Protestantism is the classic example of this (Jesus was great, Paul was greater, James was maybe a bit iffy and it mostly went downhill from there until Luther rose up to challenge the naughty Catholics and restore to the Church all the goodness that had been lost). But you find it elsewhere too: some feminist theologians argue that Jesus was pro-women, but the Church has slowly and systematically misogynised his original message, and so we need to recover the original, feminist, gospel.
3) The 'adaptive' model thinks that Christianity, the church and theology have changed over time, but, well, it's complicated. Some changes were good, some were bad, and some were indifferent. Some people think that, overall, there's a general good development (some changes weren't great, but overall the Church is getting better all the time), either because the Holy Spirit was guiding the Church or because they have a sort of 'evolutionary' idea of history, where progress rules and history is moving inexorably towards its final perfection. Some people think that we win some, we lose some, and overall there's no movement, just more or successful attempts to be the Church God intended us to be.
Ludlow says that these three models make important differences to the way we read theological texts because they shape the way we think about doctrinal authority. If you have a genuinely static view of Christian history, you won't bother reading old theology because it'll be basically the same as new theology, right? If you hold the reformatory view, you might think that old theology is useful because it's closer to the original source of the gospels, or because you want to paint yourself as part of a long tradition in the Church of people who resisted the Bad Developments. If you think that doctrine develops over time, and does so for the better, old theology is only really useful for showing how we got where we are now.
Ludlow suggests that the most productive model for reading old theology is the adaptive model which doesn't think there's been an overall improvement or degeneration in theology through history, because this model means that, at any point in the history of the Church, people might have had great ideas which might be useful for us, now. On this view also, old theologians aren't authoritative because they represent a particular tradition or because they belong to some theological 'golden age' but only insofar as their ideas are good.