Is it necessary to read Aristotle's books to understand the Summa?

[Geremia]

This question was asked on Christianity StackExchange. To it, I responded:

Although St. Thomas Aquinas is said to have "reconciled" Aristotle with Christian theology, he rejects much of Aristotle (e.g., he rejects that Aristotle thought the world is eternal).
^{(Interestingly, St. Thomas refers to Aristotle as "Aristotle" when he disagrees with him and as "The Philosopher" when he agrees with him.)}
Understand Scholastic terminology.
St. Thomas can be understood without reading any Aristotle; however, it is necessary to understand the Scholastic terminology he uses. To aid with this, see:

• this list of scholastic terms (cf. this list of scholastic axioms)
• Roy J. DeFerrari's A Lexicon of St. Thomas Aquinas (1948)

Understand the doctrine of actuality and potentiality.
Philosophically, hylemorphism (the doctrine of actuality and potentiality) forms the basis of Thomism. See:

• St. Thomas's opusculum ("short work") On the Principles of Natures (De Principii Naturæ)
  
  • ^{This was written at the request of Br. Sylvester, who wanted St. Thomas to summarize the natural philosophy that undergirds his works so students wouldn't have to spend too much time reading Aristotle Physics, which St. Thomas also commentated.}
• Fr. Réginald Garrigou-Lagrange, O.P.'s short work The Essence & Topicality of Thomism (e-book) and the longer Reality: A Synthesis of Thomistic Thought (free online ed.), esp. "Chapter 5: Act And Potency"
  
  • ^{cf. my review / summary of Reality}
• The 24 Thomistic Theses

Reading Aristotle not necessary but could be helpful
If you want to read Aristotle (it certainly doesn't hurt in helping one understand St. Thomas), read the following works by Aristotle alongside St. Thomas's commentaries on them:

• logical work: Posterior Analytics with St. Thomas's commentary
  
  • ^{cf. this commentary on St. Thomas's commentary ☺}
• natural philosophy work: Physics with St. Thomas's commentary
  
  • ^{The last book of this influenced St. Thomas's quinque viæ proofs of God's existence.}