The second, more universally accessible way, is through self-authenticating, divine truth, as he calls it. Piper basically claims that there is something about Jesus--his attitudes, thoughts, actions--in scripture that causes us to know who he was and is, without having had to learn it. He says, "it is like seeing the sun and knowing that it is light and not dark, or like tasting honey and knowing that it is sweet and not sour. There is no long chain of reasoning from premises to conclusions. There is a direct apprehension that this person is true and his glory is the glory of God."
Of course, many people read the Gospels and never believe. There are theologically liberal seminaries full of experts on these biographical accounts, yet these experts believe them to be but fables. Yes, the words and works of Jesus were and are self-authenticating, but it is a spiritual business to believe, and the Holy Spirit must open our eyes to see and experience what is real. This enabling to see reality for what it really is is quite important, because salvation doesn't depend on having correct doctrine.
Imagine that you were born blind, and that people had always lied to you and said the sky was yellow. If one day, some sympathetic person told you that everyone had lied to you, and that, in fact, the sky was blue, and for whatever reason you believed them over all the others, you still wouldn't really know what color the sky is until you saw it for yourself. After all, there are many people who confess with their lips that Jesus is the Son of God, but who don't know he is the Son of God. Knowing this truth means more than just saying the right proposition, it means that the Spirit of God has opened your eyes to SEE that He is the Son of God.