I. From ST I.46.1c, the Dominican Fathers' translation (www.summatheologiae.org), with Don Paco's revisions:
I answer that, Nothing except God existed from eternity. And this is not impossible to hold: for it has been shown above (ST 19.4) that the will of God is the cause of things. Therefore things are necessary to the extent that it is necessary for God to will them, since the necessity of the effect depends on the necessity of the cause (Metaphysics V.6). Now it was shown above (ST 19.3), that, absolutely speaking, it is not necessary for God to will anything except Himself. It is not, therefore, necessary for God to will the world to have always existed. But the world exists as long as God wills it to exist, since the existence of the world depends on the will of God as on its cause. It is not, therefore, necessary for the world to exist always; and hence it cannot be proved by demonstration.
II. Latin Original (from www.corpusthomisticum.org):
Respondeo dicendum nihil praeter Deum ab aeterno fuisse. Et hoc quidem ponere non est impossibile. Ostensum est enim supra quod voluntas Dei est causa rerum. Sic ergo aliqua necesse est esse, sicut necesse est Deum velle illa, cum necessitas effectus ex necessitate causae dependeat, ut dicitur in V Metaphys. Ostensum est autem supra quod, absolute loquendo, non est necesse Deum velle aliquid nisi seipsum. Non est ergo necessarium Deum velle quod mundus fuerit semper. Sed eatenus mundus est, quatenus Deus vult illum esse, cum esse mundi ex voluntate Dei dependeat sicut ex sua causa. Non est igitur necessarium mundum semper esse. Unde nec demonstrative probari potest.
III. The Argument in Syllogistic Format:
Major: Only that which is necessarily the case can be demonstrated.
Minor: The eternity of the world is not necessarily the case.
Conclusion: Therefore, the eternity of the world cannot be demonstrated.
[Proof of the Major: It follows from the notion of demonstration as an argument that is based on necessity. Cf. Posterior Analytics I.4.]
Proof of the Minor: Only that which God necessarily wills is necessarily the case. But God does not necessarily will that the world be eternal--he can (and Revelation tells us he does) will the world to have a temporal beginning. Therefore.