11 “Our friend Lazarus sleeps; but I go, that I may awake him out of sleep. Then said His disciples”— according to their understanding they replied— “Lord, if he sleep, he shall do well.” For the sleep of the sick is usually a sign of returning health. “Howbeit Jesus spoke of his death, but they thought that He spoke of the taking of rest in sleep. Then said Jesus unto them plainly,”— for He said somewhat obscurely, “He sleeps;”— therefore He said plainly, “Lazarus is dead.
And I am glad for your sakes that I was not there, to the intent ye may believe.” I even know that he is dead, and I was not there: for he had been reported not as dead, but sick. But what could remain hid from Him who had created it, and into whose hands the soul of the dying man had departed? This is why He said, “I am glad for your sakes that I was not there, to the intent ye may believe;” that they might now begin to wonder that the Lord could assert his death, which He had neither seen nor heard of.
For here we ought specially to bear in mind that as yet the disciples themselves, who already believed in Him, had their faith built up by miracles: not that a faith, utterly wanting till then, might begin to exist; but that what had previously come into being might be increased; although He made use of such an expression as if only then they would begin to believe. For He said not, “I am glad for your sakes,” that your faith may be increased or confirmed; but, “that you may believe;” which is to be understood as meaning, that your faith may be fuller and more vigorous.
Source: Tractates on the Gospel of John (New Advent)