Custodi, Domine, quaesumus,
Ecclesiam tuam propitiatione perpetua,
et quia sine te labitur humana mortalitas,
tuis semper auxiliis et abstrahatur a noxis,
et ad salutaria dirigatur.
Propitiatio in its fundamental meaning meanings and "an appeasing, atonement, propitiation". The dictionary of liturgical Latin Blaise also gives us a view of the word as "favor". This makes sense. God has been appeased and rendered favorable again towards us sinners by the propitiatory actions Christ fulfilled on the Cross. We have faithfully (?) renewed these through the centuries in Holy Mass. Mortalitas refers, as you might guess, to the fact that we die, our mortality. Inherent in the word is the concept that we die in our flesh. So, you ought also to hear "flesh" when you hear mortalitas.
Labitur is from labor. This is not the substantive labor but the verb, labor, lapsus. It means, "to glide, fall], to move gently along a smooth surface, to fall, slide; to slide, slip, or glide down, to fall down, to sink as the beginning of a fall".
Auxilium, in the plural, has a military overtone. There is also a medical undertone too, "an antidote, remedy, in the most extended sense of the word". Pair this up with noxius, a, um, which points at things which are injurious or harmful. There is a moral element as well or "a fault, offence, trespass".
Salutare, is (n) is "salvation, deliverance, health" in later Latin and the Vulgate. This is a very interesting word, which I wrote about at length in one of my weekly columns. You remember. That was the time I put the English version into Shakespearean iambic pentameter. Suffice to say, a "health" is like a "toast".
Guard your Church, O Lord, we beseech You,
with perpetual favor,
and since without You our mortal flesh slides toward ruin
by means of your helping remedies let it be pulled back from injuries
and be guided unto saving healths.
There are different ways to do this, but I wanted to place in evidence the image of health and the flesh and medicine.
One of the most important Patristic Christological images in the ancient Church is Christus Medicus, the "Physician". He is the doctor of the ailing soul. St. Augustine does amazing things with this image.