If you walk out of Mass and Church with the Host in your mouth, have you heard Mass?
I will answer this question in two ways: 1 – as literally given; and 2 – (at the risk of misinterpreting this question), as a rephrased question to the effect of “Is it wrong to leave Mass early?”
To walk out of Mass while still in the process of receiving Holy Communion (presumably not to return) may not seem like a big deal. One who has this habit might say, “I mean, I was there for the most important part, wasn’t I?” However, I want to invite us from the point of view of love for God to really think about what such an action really says. While it is God’s will to give us the gift of the Eucharist, this gift is given in a loving exchange, wherein we are invited to respond to His perfect gift of love with humble praise and thanksgiving to him. When our approach to receiving Holy Communion is reduced to simply “getting it done” both our response to the gift is less than what is fitting (most likely with little or no humility) and our disposition to receive God’s grace into our soul is changed in an unfortunate way. Here’s an image for us to consider from the view of a “giver” of gifts: if you give a gift to a child who never says “thank you” but takes the gift and runs without acknowledging you, your sense as a gift giver may change from one of joy in giving the gift to one of wanting to “teach them a lesson,” so to speak. If we truly love God, the giver of the greatest gifts, why would we who receive His gifts ever think of not responding to Him in the most loving way possible - which for Holy Communion minimally includes remaining at Mass through the final blessing, but more beautifully invites us to pray in thanksgiving to God throughout the rest of the Mass and even after Mass (such as by remaining in the pew to quietly pray for a little while after Mass is over)?
As for the rephrased question on leaving Mass early, the short answer is yes, it is wrong to leave Mass early. For the Mass, by its very nature is a complete prayer,
inviting us to enter fully into Communion, ahead of formally being “sent” by the final dismissal to, “Glorify the Lord by your life,” or to, “Announce the Gospel of the Lord.” This sending includes the reception of the final blessing, giving us even greater strength to live in the Communion that we have just received. Thus, it is plain that in attending Mass, proper participation means participating in the entire Mass.
Before the Deacon reads the Gospel he turns and faces you and you with your hands held over his head say something to him before making the Sign of the Cross. What are you saying to him?
Prior to reading the Gospel, he who is to read the Gospel is to ask God for his heart and lips to be clean so as to rightly proclaim the Gospel. When there is no
deacon, the priest bows before the altar praying for this grace with the words “Cleanse my heart and my lips, almighty God, that I may worthily proclaim your holy Gospel.” If there is a deacon to proclaim the Gospel, the deacon is to first ask the priest for the blessing, who responds to the deacon (while stretching out his hands over him), “May the Lord be in your heart and on your lips that you may proclaim His Gospel worthily and well, in the name of the Father and of the Son and of the Holy Spirit.”