How Can I Tell for Certain that There Ever Was Such a Man (as Jesus)?

Robert Falconer has finally taken the time to study the New Testament for himself, inspired by the words 'Peace I leave with you, my peace I give unto you: not as the world giveth give I unto you. Let not your heart be troubled, neither let it be afraid.' Here is what George MacDonald writes: 

He did not leave the place for six weeks. Every day he went to the burn, as he called it, with his New Testament; every day tried yet again to make out something more of what the Saviour meant. By the end of the month it had dawned upon him, he hardly knew how, that the peace of Jesus (although, of course, he could not know what it was like till he had it) must have been a peace that came from the doing of the will of his Father. From the account he gave of the discoveries he then made, I venture to represent them in the driest and most exact form that I can find they will admit of. When I use the word discoveries, I need hardly say that I use it with reference to Falconer and his previous knowledge. They were these: that Jesus taught—

First—That a man's business is to do the will of God:

Second—That God takes upon himself the care of the man:

Third—Therefore, that a man must never be afraid of anything; and so,

Fourth—be left free to love God with all his heart, and his neighbour as himself.

But one day, his thoughts having cleared themselves a little upon these points, a new set of questions arose with sudden inundation—comprised in these two:—

'How can I tell for certain that there ever was such a man? How am I to be sure that such as he says is the mind of the maker of these glaciers and butterflies?'

All this time he was in the wilderness as much as Moses at the back of Horeb, or St. Paul when he vanishes in Arabia: and he did nothing but read the four gospels and ponder over them. Therefore it is not surprising that he should have already become so familiar with the gospel story, that the moment these questions appeared, the following words should dart to the forefront of his consciousness to meet them:—

'If any man will do his will, he shall know of the doctrine, whether it be of God, or whether I speak of myself.'

Here was a word of Jesus himself, announcing the one means of arriving at a conviction of the truth or falsehood of all that he said, namely, the doing of the will of God by the man who would arrive at such conviction.