AP: This is a broad question. I would need to write a book just to answer it. Let me suggest two of my books that are available in electronic format:
Piñero, Antonio. Los cristianismos derrotados: ¿Cuál fue el pensamiento de los primeros cristianos, heréticos y heterodoxos? Madrid: Edaf., 2007. (325 pp. ISBN: 978-84-414-2005-2).
Piñero, Antonio. Guía para entender el Nuevo Testamento. Fourth edition. Madrid: Trotta, 2011. (568 pp. ISBN: 84-8164-832-9)_____
TWH: I'd point you to the New Testament corpus itself. A careful study of the New Testament itself will yield a great deal of information, for example, about who Jesus is, what the gospel is, what the relationship of the Mosaic Law is to the New Covenant community, and how the apostles responded to various expressions of maligned theology, especially twisted Christologies. It might sound simple, but I always point people to the Bible. You can study it your whole life and still have only touched the surface of the treasures it contains.