The Complete Ante-Nicene Fathers Collection: with linked footnotes

The Ante-Nicene Period (literally meaning "before Nicaea") in the history of early Christianity was the era following the Apostolic Age of the first century until the First Council of Nicaea in 325. During this period the orthodoxy (right belief) of the faith developed. These key authors of the faith are known as the church fathers. They not only defined the faith but wrote against various heresies.

Some of the unorthodox beliefs were:

Gnosticism (2nd to 4th centuries) – reliance on revealed knowledge from an unknowable God, a distinct divinity from the Demiurge who created and oversees the material world.
Marcionism (2nd century) – the God of Jesus was a different God from the God of the Old Testament.
Montanism (2nd century) – relied on prophetic revelations from the Holy Spirit.
Adoptionism (2nd century) – Jesus was not born the Son of God, but was adopted at his baptism, resurrection or ascension.
Docetism (2nd to 3rd century) – Jesus was pure spirit and his physical form an illusion.
Sabellianism (3rd century) – the Father, Son, and Holy Spirit are three modes of the one God and not the three separate persons of the Trinity.
Arianism (3rd to 4th century) – Jesus, as the Son, was subordinate to God the Father.

In this collected work the following volumes are presented under the editorship of Philip Schaff:

Volume I - The Apostolic Fathers with Justin Martyr and Irenaeus
Volume II - Fathers of the Second Century
Volume III - Latin Christianity: Its Founder, Tertullian
Volume IV - The Fathers of the Third Century
Volume V - The Fathers of the Third Century
Volume VI - The Fathers of the Third Century
Volume VII - The Fathers of the Third and Fourth Centuries
Volume VIII - The Fathers of the Third and Fourth Centuries
Volume IX - Recently Discovered Additions to Early Christian Literature; Commentaries of Origen

All the works presented here have in-line footnotes linked to the appropriate note at the back of the work.