Athanasius was not a speculative theologian. As he stated in his First Letters to Serapion, he held on to "the tradition, teaching, and faith proclaimed by the apostles and guarded by the fathers."[9] He held that not only was the Son of God consubstantial with the Father, but so was the Holy Spirit, which had a great deal of influence in the development of later doctrines regarding the Trinity.[9]
Athanasius's "Letter Concerning the Decrees of the Council of Nicaea" (De Decretis), is an important historical as well as theological account of the proceedings of that council, and another letter from 367 is the first known listing of all those books now accepted as the New Testament.[9] (Earlier similar lists vary by the omission or addition of a few books.)
Examples of Athanasius's polemical writings against his theological opponents include Orations Against the Arians, his defence of the divinity of the Holy Spirit (Letters to Serapion in the 360s, and On the Holy Spirit), against Macedonianism and On the Incarnation.