The structure of the Summa theologiae is beautifully described by the Cambridge theologian Nicholas Lash, who writes:
“We might almost say that, for Aquinas, the ‘soundness’ of his ‘educational method’ depended upon the extent to which the movement of his exposition reflected the rhythm of God’s own act and movement: that self-movement ‘outwards’ from divine simplicity to the utterance of the Word and breathing of the Gift which God is, to the ‘overflowing’ of God’s goodness in the work of his creation (‘Prima pars’); the ‘return’ to God along that one way of the world’s healing which is Christ (‘Tertia pars’); and, because there lies across this movement the shadow of the mystery of sin, we find, between his treatment of the whence and whither, the ‘outgoing’ and ‘return’ of creaturely existence, the drama of conversion, of sin and virtue, of rejection or acceptance of God’s grace (‘Secunda pars’). And this by way of explanation of how in a summary of Christian theology, Christ can make a central appearance only towards the end.”
—The Beginning and End of Religion. (Quoted by Aidan Nichols in Discovering Aquinas)
