live in the world, as if we were strangers and pilgrims, not citizens.

The Art of Dying Well - St. Robert Bellarmine:

Since, then, such is the truth, if we wish to learn the Art of dying well, it is our bounden and serious
duty to go forth from the world, not in word and in tongue, but in deed and in truth: yea, to die to
the world, and to exclaim with the Apostle, "The world is crucified to me, and I to the world." This
business is no trifling matter, but one of the utmost difficulty and importance: for our Lord being
asked, "Are they few that are saved?" replied, " Strive to enter by the narrow gate ;" and more clearly
in St. Matthew doth He speak: “Enter ye in at the narrow gate: for wide is the gate and broad is the
way that leadeth to destruction, and many there are who go in thereat. How narrow is the gate, and
strait is the way that leadeth to life: and few there are that find it!" (chap, vii.)

To live in the world, and to despise the pleasures of the world, is very difficult: to see beautiful
objects, and not to love them; to taste sweet things, and not to be delighted with them; to despise
honours, to court labours, willingly to occupy the lowest place, to yield the highest to all others in
fine, to live in the flesh as if not having flesh, this seems rather to belong to angels than to men; and
yet the apostle, writing to the Church of the Corinthians, in which nearly all lived with their wives,
and who were therefore neither clerics, nor monks, nor anchorets, but, according to the expression
now used, were seculars still, he thus addresses them: "This therefore I say, brethren, the time is
short; it remaineth, that they also who have wives be as if they had none; and they that weep, as
though they wept not; and they that rejoice, as if they rejoiced not; and they that buy, as though they
possessed not; and they that use this world, as if they used it not, for the fashion [figure] of this world
passeth away." (1 Corinth, vii. 29. & c.)

By these words the apostle exhorts the faithful that, being encouraged by the hope of eternal
happiness, they should be as little affected by earthly things as if they did not belong to them; that
they should love their wives only with a moderated love, as if they had them not; that if they wept
for the loss of children or of their goods, they should weep but little, as if they were not sorrowful;
that if they rejoiced at their worldly honours or success, they should rejoice as if they had no occasion
to rejoice that is, as if joy did not belong to them; that if they bought a house or field, they should be
as little affected by it as if they did not possess it. In fine, the apostle orders us so to live in the world,
as if we were strangers and pilgrims, not citizens.