In what follows, some may imagine that he says something plausible against us. “If,” says he, “these people worshipped one God alone, and no other, they would perhaps have some valid argument against the worship of others. But they pay excessive reverence to one who has but lately appeared among men, and they think it no offense against God if they worship also His servant.” To this we reply, that if Celsus had known that saying, “I and My Father are one,” and the words used in prayer by the Son of God, “As You and I are one,” he would not have supposed that we worship any other besides Him who is the Supreme God.
“For,” says He, “My Father is in Me, and I in Him.” And if any should from these words be afraid of our going over to the side of those who deny that the Father and the Son are two persons, let him weigh that passage, “And the multitude of them that believed were of one heart and of one soul,” that he may understand the meaning of the saying, “I and My Father are one.” We worship one God, the Father and the Son, therefore, as we have explained; and our argument against the worship of other gods still continues valid.
And we do not “reverence beyond measure one who has but lately appeared,” as though He did not exist before; for we believe Himself when He says, “Before Abraham was, I am.” Again He says, “I am the truth;” and surely none of us is so simple as to suppose that truth did not exist before the time when Christ appeared. We worship, therefore, the Father of truth, and the Son, who is the truth; and these, while they are two, considered as persons or subsistences, are one in unity of thought, in harmony and in identity of will. So entirely are they one, that he who has seen the Son, “who is the brightness of God's glory, and the express image of His person,” has seen in Him who is the image of God, God Himself.
Source: Against Celsus (New Advent)