With difficulty, yes, in that he could decipher the actual words being spoken after some practice. But he would have no context on the Christian terminology.

The wording of St Jerome's Vulgate is pretty simplistic Latin in form. It's not Cicero. The grammar is pretty easy and it was intentionally so: it was formulated to be understandable to the lay clergy in the Vulgar Latin West. We can assume he would be okay with sentence structure if he could read it (but written script had changed by this time, so without assistance he could not. Compare Carolingian Miniscule to the Old Roman Cursive ).

The accent would be difficult for him, and this would be dependent on who was giving the mass: a local parish priest, assuming he were educated enough to give a Latin Mass, would speak the Latin with his own accent (the Middle English of Chaucer). As English had not yet undergone the Great Vowel Shift, this may not have been too bad, but it would be quite a shock at first.

A Bishop giving the mass, if educated in Rome, would have had more of an Italian pronunciation (if I may make a bold generalization, as every town in Italy had its own variation, and still does). Things like the palletization of C would come into play, as the pronunciation of Latin in Italy had changed: words like Caesar would be a tad confusing for our dear senator. He would still probably expect a hard K sound at the start rather than a soft CH. And that diphthong ae? That's now pronounced e. So, Cesare. Pretty different.

With some practice he would likely understand the words being spoken. But as I said, he would have absolutely no context on the vocabulary in play. What would you make of transubstantiation if you knew nothing about Christianity? You'd hear the word but the meaning would be lost on you. A Roman senator of the second century would have no real knowledge of Christian ritual beyond what he'd heard of it from rumors: it was still a persecuted cult.

In short, after some exposure, the Senator would probably be able to understand the Latin, but he wouldn't like the sound of it very much. However he would be completely lost with regard to the terminology used in the sermon.