The Gospel reading at Mass was John 20:19-31. Thomas, after the Risen Lord asked him to see the mark of the nails in his hands and put his finger into the nail-marks and put his hand into his side, believed and answered to Jesus, "My Lord and my God!" What a most beautiful response! Catholics were taught to address the Lord thus right after the consecration of the bread at Mass. If your heart feels like exclaiming it whenever Jesus comes to your mind, you can be sure that it's the Holy Spirit who's prompting you to do so.