The objection is serious only if Muslims are claiming that the present Torah and Gospel are wholly identical with the revelations originally given to Moses and Jesus. Islam makes a more careful distinction: Muslims believe in every revelation from God, but they do not treat every surviving text, interpretation, or theological claim as preserved revelation.
What Quran 2:85 is actually condemning
The verse addresses the Children of Israel who accepted some divine commands while knowingly violating others, especially the covenant's obligations concerning one another. They were not condemned merely for distinguishing truth from falsehood. They were condemned for accepting the law when it suited them and abandoning it when it demanded sacrifice, even though they claimed allegiance to the same covenant.
Then you are those who kill one another, evicting a group of your people from their homes, cooperating against them in sin and aggression. And if they come to you as captives, you ransom them, although their expulsion was forbidden to you. Do you believe in part of the Scripture and reject the rest?
Quran 2:85
The wording and context concern selective obedience to a single binding revelation, not the belief that God sent several revelations and that later claims about them must all be accepted without examination. A Muslim who accepts God's original revelation to Moses and Jesus, while rejecting later additions or interpretations that contradict the Quran, is not doing what this verse describes.
What Muslims believe about the earlier scriptures
The Quran repeatedly affirms the divine origin of the Torah and Gospel and describes the Quran as confirming the truth that remains in them. It also criticizes some People of the Scripture for concealing, altering, or misrepresenting revelation. Thus Islam neither says, "All previous scripture is false," nor says, "Everything currently attributed to it must be accepted."
Believe in what I have revealed, confirming what is with you, and do not be the first to deny it.
Quran 2:41
We have revealed to you the Book in truth, confirming the Scriptures that came before it and as a criterion over them.
Quran 5:48
The phrase "as a criterion over them" is the needed principle. The Quran authorizes itself to confirm what is genuine and judge disputed claims. This is not arbitrary picking and choosing; it is accepting the earlier revelation in principle while allowing God's final revelation to identify what has been preserved, altered, or misunderstood.
What this argument does and does not establish
The objection does establish that Muslims cannot dismiss the previous revelations wholesale, and it rightly demands consistency. Islam meets that demand: Muslims must believe in the Torah and Gospel as revelations from God, just as the Quran says it confirms what was with the earlier communities (2:89). But the argument does not establish that every doctrine found in later Christian scripture or theology was part of the original Gospel. In particular, the Quran explicitly rejects divine sonship and the deification of Jesus while honoring him as a mighty messenger.
So Quran 2:85 condemns selective obedience to known revelation, not the Quranic method of judging earlier texts by the final revelation. It establishes a warning against hypocrisy, not a requirement to accept every later claim made in the name of scripture.