The Account of Doubting Thomas can be found here in the NRSV translation:

John 20:19-31

In sermons on this passage Thomas often gets all the attention (e.g. What does it mean to doubt?  Is it okay to Doubt?  Why it is okay to doubt), but this passage can also be a wonderful primer on and encouragement for Evangelism.  

Consider, after all, that the Evangelist uses this account to just such an end.  For John this is an evangelism story.  It leads directly to John's own personal exhortation in his gospel for people to believe.

Consider how this text can be for us an Evangelism primer: 

First the disciples themselves encounter Christ just as many of us have or did.  Like many who are first receiving faith they go through the fear (Jesus has to say "peace be with you") and exultation (now settling them down he says, "peace be with you"), and then they receive his Holy Spirit.  

Having become believers one of the first things they do is tell their good friend Thomas.  They do not harangue him or twist his arm.  Simply they tell him the wonderful truth from their own experience, "We have seen the Lord!" 

Note that just as frequently happens in real life Thomas does not suddenly and instantly believe because of their words.  There is a season, a time, in this case a week of days in which Thomas continues to disbelieve.  

However, they have planted a seed.  He knows their faith.  Surely he sees the activity of faith in them.  But Thomas doubts during this time.

Note here that Thomas has specific doubts.  For him to believe he would need to see the marks of the nails and the wound in his side.  In time Christ comes to Thomas.  And in coming, Christ also speaks directly and personally to the needs of Thomas.  Addressing his particular doubts.  

Thomas comes to faith, but not by what the disciples did, but by what Jesus himself did in him.  The work of conversion is God's.  Our role is significant, but merely one part in a person's coming to saving faith.

At this point things become even more interesting.  As Thomas comes to faith he cries out "My Lord and my God!"  He has gone a step further than the disciples in this!  They had said to him, "We have seen the Lord!"  But Thomas, in coming to faith, now has something wonderful to share back to the disciples.  And that is just how it is, isn't it?  Through the Body of Faith each of us has something important and wonderful to share with the other, however long we or they have been believers.  

So the whole body is built up by God through Christ, by proclamation and belief.

I've heard some fine sermons on this passage about the pros and cons of doubting and Thomas as model or warning.  On this particular day, however, I find the above a much more interesting read!