Eusebius of Caesarea is not to be confused with Eusebius of Nicomedia, two contemporaries who were sympathetic to Arianism, or with St. Eusebius of Samosata, a later champion in the 4th century against Arianism.
Eusebius of Caesarea was certainly very learned and the Church does find much of his Ecclesiastical History useful and of value, however, he was condemned in the 7th Ecumenical Council for his Arian views.
Many of his writings have been preserved, and one can still find Arian language in a number of them.
From the Acts of the Seventh Oecumenical Council.
"For who of the faithful ones in the Church, and who of those who have obtained a knowledge of true doctrine, does not know that Eusebius Pamphili has given himself over to false ways of thinking, and has become of the same opinion and of the same mind with those who follow after the opinions of Arius? In all his historical books he calls the Son and Word of God a creature, a servant, and to be adored as second in rank. But if any speaking in his defense say that he subscribed in the council, we may admit that that is true; but while with his lips he has respected the truth, in his heart he is far from it, as all his writings and epistles go to show. But if from time to time, on account of circumstances or from different causes, he has become confused or has changed around, sometimes praising those who hold to the doctrines of Arius, and at other times reigning the truth, he shows himself to be, according to James the brother of our Lord, a double-minded man, unstable in all his ways; and let him not think that he shall receive anything of the Lord. For if with the heart he had believed unto righteousness, and with the mouth had confessed the truth unto salvation, he would have asked forgiveness for his writings, at the same time correcting them. But this he has by no means done, for he remained like Aethiops with his skin unchanged. In interpreting the verse I said to the Lord, Thou art my Lord,' he has strayed far away from the true sense, for this is what he says:
By the laws of nature every son's father must be his lord; wherefore God who begat him must be at the same time God, Lord, and Father of the only-begotten Son of God.' So also in his epistle to the holy Alexander, the teacher of the great Athanasius, which begins thus: With what anxiety and with what care have I set about writing this letter,' in most open blasphemy he speaks as follows concerning Arius and his followers:
Thy letter accuses them of saying that the Son was made out of nothing, like all men. But they have produced their own epistle which they wrote to thee, in which they give an account of their faith, and expressly confess that "the God of the law and of the prophets and of the New Testament, before eternal ages begat an only-begotten Son, through whom also he made the ages and the universe; and that he begat him not in appearance, but in truth, and subjected him to his own will, unchangeable and immutable, a perfect creature of God, but not as one of the creatures." If, therefore, the letter received from them tells the truth, they wholly contradict thee, in that they confess that the Son of God who existed before eternal ages, and through whom he made the world, is unchangeable and a perfect creature of God, but not as one of the creatures. But thy epistle accuses them of saying that the Son was made as one of the creatures. They do not say this, but clearly declare that he was not as one of the creatures. See if cause is not immediately given them again to attack and to misrepresent whatever they please. Again thou findest fault with them for saying that He who is begat him who was not. I wonder if any one is able to say anything else than that. For if He who is is one, it is plain that everything has been made by Him and after Him. But if He who is is not the only one, but there was also a Son existing, how did He who is beget him who was existing? For thus those existing would be two.' These things then Eusebius wrote to the illustrious Alexander; but there are also other epistles of his directed to the same holy man, in which are found various blasphemies in defense of the followers of Arius. So also, in writing to the bishop Euphration, he blasphemes most openly; his letter begins thus: I return to my Lord all thanks'; and farther on:
For we do not say that the Son was with the Father, but that the Father was before the Son. But the Son of God himself, knowing well that he was greater than all, and knowing that he was other than the Father, and less than and subject to Him, very piously teaches this to us also when he says, "The Father who sent me is greater than I."' And farther on: `Since the Son also is himself God, but not true God.' So then from these writings of his he shows that he holds to the doctrines of Arius and his followers. And with this rebellious heresy of theirs the inventors of that Arian madness hold to one nature in hypostatic union, and affirm that our Lord took upon himself a body without soul, in his scheme of redemption, affirming that the divine nature supplied the purposes and movements of the soul: that, as Gregory the Divine says, they may ascribe suffering to the Deity; and it is evident that those who ascribe suffering to the Deity are Patripassians. Those who share in this heresy do not allow images, as the impious Severus did not, and Peter Cnapheus, and Philoxenus of Hierapolis, and all their followers, the many-headed yet headless hydra. So then Eusebius, who belongs to this faction, as has been shown from his epistles and historical writings, as a Patripassian rejected the image of Christ," etc.