Saint Augustine was not certain whether the deliberate assertion of a falsehood is still a lie if the person does not intend to deceive.
Saint Augustine: “But whether a lie be at some times useful, is a much greater and more concerning question. Whether, as above, it be a lie, when a person has no will to deceive, or even makes it his business that the person to whom he says a thing shall not be deceived although he did wish the thing itself which he uttered to be false, but this on purpose that he might cause a truth to be believed; whether, again, it be a lie when a person willingly utters even a truth for the purpose of deceiving; this may be doubted. But none doubts that it is a lie when a person willingly utters a falsehood for the purpose of deceiving: wherefore a false utterance put forth with will to deceive is manifestly a lie. But whether this alone be a lie, is another question. Meanwhile, taking this kind of lie, in which all agree, let us inquire, whether it be sometimes useful to utter a falsehood with will to deceive.” [On Lying, first paragraph, n. 5.]
However, in the same article, Augustine counsels us neither to assert falsehoods nor to deceive [On Lying, n. 4, last sentence].
Saint Aquinas believed that deliberately asserting a falsehood was a lie, even without the intention to deceive: “The desire to deceive belongs to the perfection of lying, but not to its species, as neither does any effect belong to the species of its cause.” [Summa Theologica, II-II, 110, 3, Reply to Objection 3. ]
What Aquinas is saying is that the deliberate assertion of a falsehood is a lie, and if you add to that the intention to deceive, it is more sinful. An act wrong by its “species” (or nature) is intrinsically evil and therefore always wrong.
In answer to the question “Whether every lie is a sin”, Thomas writes: "Therefore every lie is a sin, as also Augustine declares (Contra Mend. i). "