Here is the quote from Alford's Greek Testament:
"No countenance is given by this verse to the ancient Pelagian or the modern heresy of perfectibility in this life. Such a sense of the words would be utterly at variance with the whole of the discourse. See especially vv. 22, 29, 32, in which the imperfections and conflicts of the Christian are fully recognized. Nor, if we consider this verse as a solemn conclusion of the second part of the Sermon, does it any the more admit of this view, asserting as it does that likeness to God in inward purity, love, and holiness, must be the continual aim and end of the Christian in all the departments of his moral life" (Volume I:48).