3 We are therefore to understand that their faith in Him was not yet such as He wished it to be, inasmuch as even Peter himself, to whom, on making the confession, “You are the Christ, the Son of the living God,” He had borne so excellent a testimony, was disposed rather to hinder Him from dying than to believe in His resurrection when dead, and hence was called immediately thereafter by the same of Satan. Those, accordingly, are found to be the greater in faith who were long since deceased, and yet, through the revelation of the Spirit, had no manner of doubt that Christ would rise again, than those who, after attaining to the belief that He should redeem Israel, at the sight of His death lost all the hope they previously possessed regarding Him.
The best thing for us, therefore, to believe is, that after His resurrection, when the Holy Spirit was bestowed, and the apostles taught and confirmed, and from its outset constituted teachers in the Church, others, through their word, attained the proper faith in Christ, or, in other words, that they then got firm hold of the faith of His resurrection. And in this way also, that all those who seemed to have already believed on Him really belonged to the number of those for whom He prayed, when He said, “Neither pray I for these alone, but for them also who shall believe in me through their word.”
Source: Tractates on the Gospel of John (New Advent)