Although of course one ought to read City of God and Against Heresies at some point, I'm happy to tell you that there are easier places to start. One of the best and most accessible starting-points is in the great compilation put together by Olivier Clement, an Orthodox theologian from France: The Roots of Christian Mysticism: Texts from the Patristic Era with Commentary. Don't let the yawn-inducing title fool you: this book is a gem. It carefully selects standout and representative texts, usually no more than a paragraph or two in length, from all across the spectrum of the early church fathers and mothers. It also gives one a sense of the major themes of early church theology, which is helpful because the early church fathers didn't really write systematic theologies as such, and so to truly get a sense for the overall theological system, with all its characteristic emphases, one would have to read representative works from at least a dozen of the biggest names, and even that would probably still result in a limited picture. But Clement does that job for us, sorting the texts he chooses into smooth-flowing chapters with clear theological trajectories, so that sayings from Basil and Augustine and Macarius all blend together towards one point of doctrine or practice.
And if you're looking for pure, unadulterated doctrine, such as you might find in a Reformed systematic theology, you'll be disappointed. But for a very good reason--the early church fathers held to a much more integrative view of doctrine than we do, such that doctrines are inextricably connected with the mystical experience of the church as a whole and of the individual Christian soul. Thus it is The Roots of Christian Mysticism, because for the early church fathers, all doctrine was mystical in its application: it has to do with the mystery of salvation, which is still in motion, and which can be experienced by each and every one of us. Evangelicals will find insights on prayer and "the deeper life" such as they've never encountered before, and it will be an enriching experience.
To top it all off, Clement's own commentary weaves in around the quotes from the early church fathers, elucidating and beautifying the points in question. Clement's thoughts, of course, draw their inspiration from the original insights of the fathers, but in many cases it is Clement's words that put it best--he is able to draw out those ancient insights and put them in terms that are truly unforgettable.
This is not a quick or a light book to read. It is a book to be savored, and a book to be lived.