In Doubting Thomas we see the Divine Mercy of Christ revealed. Not only does Our Lord suffer and die on the cross for our redemption. Christ allows His Apostle to thrust his hand into His side.
St John is often associated with the Sacred Heart of Christ because the beloved Apostle laid his head on the chest of Christ at the Last Supper. But in St Thomas, God reveals that He will condescend to our doubts and allow the hand of a doubter penetrate into His sacred Body toward His most Sacred Heart. This is core where faith is confirmed.