What did they doubt? That the one who appeared to them was really Jesus; that he had really died and risen. But we know that Christ had already appeared to them, and they had seen him and had joy in their hearts at his resurrection. How could they doubt what they had already believed? But does faith in the resurrection, once given, never fade again? No. Faith was needed each time Christ appeared to them, that it was truly him, and that they were seeing him because he was risen. They needed to believe again the wonder of it. We must go on believing, exercising faith, and as we do so faith will grow stronger.
Matthew Henry says, ‘Though there were those that doubted, yet, he did not therefore reject them; for he will not break the bruised reed.’ He did not lose patience with them and fault them so that he would not allow them to worship any more. He came to them; he drew near to them so that they could see him and hear his words, and have their faith strengthened and their doubts driven out. They were at fault for doubting, for they failed to believe what he had said, but the words that followed would have helped them to firm up their faith.
It is suggested that the simplest explanation of this was that they did not recognise him, but surely the Scripture would not draw attention to this on the trivial basis that he was not yet near enough to be seen clearly. This is a doubt about the resurrection itself, not just a matter of doubtful vision.