Some think that it would be better to extinguish this impulse — the impulse to live — because it is dangerous. I would like to say, especially to young people: our worst enemy is not practical problems, no matter how serious and dramatic: life’s greatest danger is a poor spirit of adaptation which is neither meekness nor humility, but mediocrity, cowardice. Is a mediocre young person a youth with a future or not? No! He or she remains there, will not grow, will not have success. Mediocrity or cowardice. Those young people who are afraid of everything: ‘No, this is how I am...’. These young people will not move forward. Meekness, strength, and not cowardice, not mediocrity.
And the English for the explanatory footnote says:
The Fathers speak of cowardice (oligopsychìa). Saint John Damascene defines it as “the fear of completing an action” (Exact exposition of the Orthodox faith, ii, 15) and Saint John Climacus adds that “cowardice is a childish disposition, in an old, vainglorious soul” (Ladder of Divine Ascent, xxi, 2).
The Italian that keeps being translated as 'cowardice' is, of course, pusillanimità, pusillanimity, small-souledness. And that is what one would expect from the parenthetical oligopsychia, which means not 'cowardice' but smallness of soul and whose exact Latin counterpart is pusillanimitas.
Opus Dei has an infinitely superior translation.
(As a side note, it's somewhat interesting that, despite the existence of relevant Western discussions, almost all of the theologians referred to in the footnotes are Eastern, with St. Ignatius being the one exception.)