2. A second point, which may be regarded as certain with regard to this question, is, that _for the period which joins the age of Irenæus to the age of St. John, we have not sufficient evidence to arrive at anything like proof_. The evidence has received important additions during the present century, and still more important additions are by no means impossible; but at present our materials are still inadequate. And the evidence is insufficient in two ways. First, although surprisingly large as compared with what might have been reasonably expected, yet in itself, the literature of this period is fragmentary and scanty. Secondly, the dates of some of the most important witnesses cannot as yet be accurately determined. In many cases to be able to fix the date of a document within twenty or thirty years is quite sufficient: but this is a case in which the difference of twenty years is a really serious difference; and there is fully that amount of uncertainty as to the date of some of the writings which are our principal sources of information; _e.g._, the _Doctrine of the Twelve Apostles_, the Epistles of Ignatius, the _Shepherd of Hermas_ and the _Clementines_. Here also our position may improve. Further research may enable us to date some of these documents accurately. But, for the present, uncertainty about precise dates and general scantiness of evidence compel us to admit that with regard to many of the points connected with this question nothing that can fairly be called proof is possible respecting the interval which separates the last quarter of the first century from the last quarter of the second. This feature of the problem is sometimes represented by the useful metaphor that the history of the Church just at this period "passes through a tunnel" or "runs underground." We are in the light of day during most of the time covered by the New Testament; and we are again in the light of day directly we reach the time covered by the abundant writings of Irenæus, Clement of Alexandria, Tertullian, and others. But during the intervening period we are, not indeed in total darkness, but in a passage the obscurity of which is only slightly relieved by an occasional lamp or light-hole. Leaving this tantalizing interval, about which the one thing that is certain is that many certainties are not likely to be found in it, we pass on to look for our two next certainties in the periods which precede and follow it.