A: When the proclamation of the Gospel is complete, the General Instruction of the Roman Missal (GIRM) directs that the homily begin (n.136).  Is there to be some kind of sung acclamation following the Gospel?  Not, at least, according to the GIRM or any other liturgical document.  In Masses at which a Bishop presides, both the GIRM and the Ceremonial for Bishops indicate that “the deacon may carry the Book of the Gospels to the credence table or to another appropriate and dignified place” (n.175 and n.141, respectively).  This practice, perhaps, is the origin of some kind of accompanying music following the Gospel and before the homily.  In any case, neither the present norms nor the liturgical tradition (that I am aware of) is the origin of singing anything after the Gospel.

            But because there exists no norm or rubric prohibiting the practice, is singing “Spirit of the Living God” or another acclamation therefore permitted?  The answer in this case, and in almost every similar case, is no.  Liturgical law, almost always, is written to direct us toward what is to be done and not, at least not primarily, to keep us from doing what is not allowed;  liturgical law is positive and proscriptive, rather than negative and prohibitive.  What we may be most accustomed to in the secular sphere is law that limits our actions:  don’t speed, don’t litter, don’t shoplift.  Such “common law” does prohibit us from doing certain things.  Liturgical law, on the other hand, like Canon Law, tells us what we should do, rather than what we should not.  For example:  “The priest says:  In the name of the Father, and of the Son, and of the Holy Spirit.  The people answer, Amen” (GIRM, nl.124).  Note that the instruction is about what the priest does say and how the people are to respond.  There are a great many things that the priest could say, such as “Hello,” or “Good morning.”  Likewise, there are any number of responses the people could give after invoking the Trinity, like “I believe,” or “Hooray!”  Liturgical law, though, does not envision every possibility and then offer a specific prohibition against them (that would make for a very long General Instruction!).  Thinking, then, that because the GIRM “doesn’t say we can’t” allows for a particular practice to take place is not in keeping within the intent of the liturgical law, even though such thinking may be accurate according to modern, common law.  Perhaps the “bottom line” is that given by the Second Vatican Council in its Constitution on the Sacred Liturgy.  Its very first norm on the reform on the liturgy states:  “[N]o other person, even if he be a priest, may add, remove, or change anything in the liturgy on his own authority” (n.22). (Answered by Christopher Carstens).