An extract from William Cunningham's Historical Theology (you can buy it at an excellent price here):
We have seen, in considering Justin Martyr and Irenaeus, that even in the second century there was, besides much very inaccurate interpretation of particular passages of Scripture, some tendency manifested to deviate from the simplicity of scriptural doctrine as taught by the apostles, though not yet carried out to any considerable extent.
Since there is as much of this tendency manifested by Irenaeus, who was no philosopher, as by Justin, who was well acquainted with the literature and philosophy of paganism, we cannot trace the incipient corruption of doctrine wholly at least to the influence of philosophical speculation, or indeed to any one specific cause, except what is in some sense the proximate cause of all error and heresy,--viz., the want of due subjection to the authority of God's word, and of due diligence and impartiality in the use of the right means of attaining to a correct knowledge of its meaning.
William Cunningham, Historical Theology vol. I, p. 146-7