John 20:19-29 is where John emphasises the account of Thomas disbelief of Christ's resurrection and Jesus Christ appeared unto them in v29
Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed.
Faith is choice it is not an exercise of imagination. To believe in bible one need to see the evidences and truths that abide in scripture
1 Peter 3:16 says this is how we address world our faith.
It is not what you believe but why you believe. John was disciple among who walked with Christ and he records his evidence in gospel so that people shall hear and believe and those things that he believed and ask us to believe the same. John emphasis saying gospel is sufficient to believe in Christ.
Taking example: Judge in court has given opportunity to decide the case and for sure he is not part of that incident but still he has to take right decision based on evidences presented to him. He believes in witnesses and presented facts and then he takes the decision.
So when gospel is presented to world and people have to decide and take the gospel presented by right heart and mind not by any emotion or under pressure as it is of no profit. In same fashion John puts the facts of his eye witness and asks his hearers to judge and believe. V31
But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name. John records 7 signs, 7 statements, 7 claims of Jesus and among all greatest sign was Christ's resurrection which made us to come to him.
Mark 16:17-20 this for one who looks for sign to believe, and surely scripture states sign was to confirm the word and if we had accurately recorded the word then we no need to show the sign.
Another example: Everyone know William Shakespeare's Romeo and Juliet and all agrees that it is his plays and writing, so did this generation saw him writing? But not all aggread with evidences and records that presented to them, and the same you agree to ceasor's recorded history and many similar.
The effects of gospel which John wrote that Jesus said were forecasted and we see it came true and Bible still exists as most popular book ever and transformed many lives. John recommends his listeners to same gospel which he knows and wants all to excel in same faith.
What if John was not true?
If you look into history Thomas was martyred for preaching gospel in India and similar incidence killed rest of them. Do any one believe people make rumour and die for them? No way none had died for lie or illusions. Dying requires more moral faith than to lie. After all reading to gospel and if one hardens his heart to truth then they can't be partakers of God's family and no one can force them all we show them humbleness. People believe in Christ by amazing facts of recorded truths through there free grace which God has given unto all. John writes to make sure we are not put in the place of Thomas. **Jesus not said possibles of faith but surety of faith. Urge all to read gospel of John to make your judgement of where you would like to spend your