"There is not sufficient evidence from Scripture that one might justify the introduction of infant baptism at the time of the early Christians after the apostolic period...But so much is evident, that no one may venture with a good conscience to reject or abandon infant baptism, which has for so long a time been practiced."
If you Google around for this quote, you'll find it ultimately referenced back to The Reformers and Their Stepchildren By Leonard Verduin, pages 203-204. Verduin documents the quote as being from WA 26:67. This is an error, the quote is from WA 26:167. This can be cross-referenced over to LW 40:255
But this God would easily forgive me, since it was done in ignorance and more than that out of fear. I did not invent it. It came to me by tradition and I was persuaded by no word of Scripture that it was wrong. I would have been unwilling to do it, had I been convinced otherwise. It would be very much as when I preach the Word, also according to his command, among the unbelieving and without fruit, or as it is said, cast pearls before swine, or holy things to the dogs [Matt. 7:6]. What could I do? Here, too, I would rather sin in preaching fruitlessly than in refusing to preach at all. For in fruitless preaching I would not be guilty of a soul [being lost] while in refusing to preach I might be held accountable for many souls. That would be too much for any individual. This I say even if there were uncertainty about the faith of children in baptism, for we cannot set aside baptism which is certain, on account of faith which is uncertain. Baptism did not originate with us, but with the apostles and we should not discard or alter what cannot be discarded or altered on clear scriptural authority. God is wonderful in his works. What he does not will, he clearly witnesses to in Scripture. What is not so witnessed to there, we can accept as his work. We are guiltless and he will not mislead us. If we knew or believed that child baptism was useless, it would be a wicked thing to still baptize. So the Waldenses do, but that is to despise God and his Word.
In the second place, this is an important consideration: No heresy endures to the end, but always, as St. Peter says, soon comes to light and is revealed as disgraceful. So St. Paul mentions Jannes and Jambres and their like [II Tim. 3:8f.], whose folly is finally plain to all. Were child baptism now wrong God would certainly not have permitted it to continue so long, nor let it become so universally and thoroughly established in all Christendom, but it would sometime have gone down in disgrace. The fact that the Anabaptists now dishonor it does not mean anything final or injurious to it. Just as God has established that Christians in all the world have accepted the Bible as Bible, the Lord’s Prayer as Lord’s Prayer, and faith of a child as faith, so also he has established child baptism and kept it from being rejected while all kinds of heresies have disappeared which are much more recent and later than child baptism. This miracle of God is an indication that child baptism must be right. He has not so upheld the papacy, which also is an innovation and has never been accepted by all Christians of the world as has child baptism, the Bible, faith, or the Lord’s Prayer, etc.
You say, this does not prove that child baptism is certain. For there is no passage in Scripture for it. My answer: that is true. From Scripture we cannot clearly conclude that you could establish child baptism as a practice among the first Christians after the apostles. But you can well conclude that in our day no one may reject or neglect the practice of child baptism which has so long a tradition, since God actually not only has permitted it, but from the beginning so ordered, that it has not yet disappeared.
For where we see the work of God we should yield and believe in the same way as when we hear his Word, unless the plain Scripture tells us otherwise. I indeed am ready to let the papacy be considered as a work of God. But since Scripture is against it, I consider it as a work of God but not as a work of grace. It is a work of wrath from which to flee, as other plagues also are works of God, but works of wrath and displeasure.