30 And truly Jesus did many other signs in the presence of His disciples, which are not written in this book;
Verses 30 and 31 set forth the purpose of the gospel of John. They are also the formal conclusion to the gospel.
30 And truly Jesus did many other signs [miracles]
A sign is a miracle that points to something specific. It is a miracle with its meaning understood. In this context, it is a miracle that proves the credentials of who Christ is and His authority.
in the presence of His disciples,
The apostles witnessed most of the miracles of Jesus.
which are not written in this book;
The gospel of John does not record all the miracles that Jesus did. John records only 7 of the 35 miracles in the synoptic gospels. Jesus also performed miracles not recorded in the Bible.
God looks at those who doubt with grace.
The interaction of Jesus and Thomas addresses the issue of skepticism. God always deals in grace with those who have doubts. God constantly asks the skeptic to examine the evidence. There is nothing in the Bible about a leap into the dark. God does not require us to believe in something without an object or content. He never asks us to abandon reason, logic, or any objective means of thought. The Bible itself rests on factual information and careful thinking (logic).