18 The next words are, For what things soever He— the Father— does, these also does the Son likewise. This likewise is added to indicate His birth; whatsoever and same to indicate the true Divinity of His nature. Whatsoever and same make it impossible that there should be any actions of His that are different from or outside, the actions of the Father. Thus He, Whose nature has power to do all the same things as the Father, is included in the same nature with the Father.
But when, in contrast with this, we read that all these same things are done by the Son likewise, the fact that the works are like those of Another is fatal to the supposition that He Who does them works in isolation. Thus the same things that the Father does are all done likewise by the Son. Here we have clear proof of His true birth, and at the same time a convincing attestation of the Mystery of our faith, which, with its foundation in the Unity of the nature of God, confesses that there resides in Father and Son an indivisible Divinity.
For the Son does the same things as the Father, and does them likewise; while acting in like manner He does the same things. Two truths are combined in one proposition; that His works are done likewise proves His birth; that they are the same works proves His nature.
Source: On the Trinity (New Advent)