12 But what is this that He says: “He that abides in me, and I in him”? What, but that which the martyrs heard: “He that perseveres unto the end, the same shall be saved”? How did Saint Laurence, whose feast we celebrate today, abide in Him? He abode even to temptation, abode even to tyrannical questioning, abode even to bitterest threatening, abode even to destruction—that were a trifle, abode even to savage torture. For he was not put to death quickly, but tormented in the fire: he was allowed to live a long time; nay, not allowed to live a long time, but forced to die a slow, lingering death.
Then, in that lingering death, in those torments, because he had well eaten and well drunk, as one who had feasted on that meat, as one intoxicated with that cup, he felt not the torments. For He was there who said, “It is the Spirit that quickens.” For the flesh indeed was burning, but the Spirit was quickening the soul. He shrunk not back, and he mounted into the kingdom. But the holy martyr Xystus, whose day we celebrated five days ago, had said to him, “Mourn not, my son;” for Xystus was a bishop, he was a deacon.
“Mourn not,” said he; “you shall follow me after three days.” He said three days, meaning the interval between the day of Saint Xystus's suffering and that of Saint Laurence's suffering, which falls on today. Three days is the interval. What comfort! He says not, “Mourn not, my son; the persecution will cease, and you will be safe;” but, “do not mourn: whither I precede you shall follow; nor shall your pursuit be deferred: three days will be the interval, and you shall be with me.” He accepted the oracle, vanquished the devil, and attained to the triumph.
Source: Tractates on the Gospel of John (New Advent)