A question on the Gospel of Barnabas.

[AMuslimDude213]

Why are Christians soo skeptical about it? it was written before the birth of Muhammad PBUH,carbon dated right to the time of Barnabas,
and was a part of the Injil,but still Christians are skeptical?

[khdrb]

(2:101)"And when there cometh unto them a messenger from Allah, confirming that which they possess, a party of those who have received the Scripture fling the Scripture of Allah behind their backs as if they knew not,"

(3:187)"And (remember) when Allah laid a charge on those who had received the Scripture (He said): Ye are to expound it to mankind and not to hide it. But they flung it behind their backs and bought thereby a little gain. Verily evil is that which they have gained thereby."