The disciples had known Christ to be really man, having been so long a time with Him; but after that He was dead, they do not believe that the real flesh could rise again from the grave on the third day. They think then that they see the spirit which He gave up at His passion. Therefore it follows, But they were terrified and affrighted, and supposed that they had seen a spirit. This mistake of the Apostles was the heresy of the Manichæans.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).