Divine Institutes 4.14:
"Nor did He [Christ] at any time say that He Himself was God; for He
would not have maintained His faithfulness, if, when sent to abolish
the false gods, and to assert the existence of the one God, He had
introduced another besides that one. This would have been not to
proclaim one God, nor to do the work of Him who sent Him, but to
discharge a peculiar office for Himself, and to separate Himself from
Him whom He came to reveal. On which account, because He was so
faithful, because He arrogated nothing at all to Himself, that He
might fulfil the commands of Him who sent Him, He received the dignity
of everlasting Priest, and the honour of supreme King, and the
authority of Judge, and the name of God" (Divine Institutes 4.14).
"God, then, who invented and constructed all things, as we said in book 2,
before approaching the remarkable task of making this world CREATED a
holy and incorruptible spirit whom he called his son and though he
later created countless others, whom we call angels, this, his first-
born, was the only one he distinguished with a name of divine
significance, presumably because he had his father's qualities of
power and supremacy" (Divinae institutiones 4.6.1).
"For if God, who made all things, is also Lord and Father, He must be one
only, so that the same may be the head and source of all things. Nor
is it possible for the world to exist unless all things be referred
to one person, unless one hold the rudder, unless one guide the
reins, and, as it were, one mind direct all the members of the body"
All such utterances must be construed in accord with the literary context in which they appear. For instance, Lactantius argues that the
Father and the Son share a moral unity: "Because the son is loyal to
the supreme father and precious to him, he is never separated from
him, just as a river cannot be cut off from its source nor a sunbeam
from the sun . . ." (DI 4.29.5). See 4.29.7-8. For Lactantius, the
Father (strictly speaking) is the exclusive master over the created
order or universal household: "the world has one king, one father and
one lord only" (DI 1.7.3). The Father and Son are one in a moral or
legal sense as demonstrated in DI 4.29.7-8.
Another passage that also leads me to believe that Lactantius does not believe that the Son is fully God is the passage at DI 2.8.3-4:
"Since nothing existed at the time [of creation] apart from himself,
because the source of full and perfect good was in himself, as it
always is, in order that good should spring from him like a stream and
flow forth and on and on, he produced a spirit like himself, which was
to be endowed with all the virtues of God his father . . . Then by
means of the one he made first he made another, liable to corruption.
In this one the divine inheritance was not to abide."
Charles Thomas Cruttwell writes these words about Lactantius:
"Theologians have detected many flaws in his [Lactantius'] orthodoxy. It cannot be denied that he is unsatisfactory in his definition of the Godhead of Christ; that his theory of the part assigned to the angels in the government of mankind is unscriptural and unwarranted; and that
his omission of all mention of the Third Person of the Blessed Trinity
is a grave theological defect. In fact, as has already been stated,
his contribution to Christian dogma is of little theological value.
It is rather to his earnestness, his purity of spirit, and his
soundness of moral judgment, that he owes his high position as a
Christian writer" (A Literary History of Early Christianity Including the Fathers and the Chief Heretical Writers of the Ante-Nicene
Period, page 650).
I also offer a quote which was reproduced above, from another translation:
"in order that goodness might spring as a stream from Him, and might flow forth afar, He produced a Spirit like to Himself, who might be endowed with the perfections of God the Father. But how He willed that, I will endeavour to show in the fourth book. Then He made another being, in whom the disposition of the divine origin did not remain. " (Divine Institutes 2.9 in some editions).
"It is evident from the testimony of the poet, that there is one God who inhabits the world, since the whole body cannot be inhabited and governed except by one mind. Therefore all divine power must be in one person, by whose will and command all things are ruled; and therefore He is so great, that He cannot be described in words by man, or estimated by the senses" (De ira Dei) 11.