Some faults grow up without any natural occasion giving birth to them, but simply from the free choice of a corrupt and evil will, as envy and this very sin of covetousness; which are caught (so to speak) from without, having no origination in us from natural instincts. But these, in proportion as they are easily guarded against and readily avoided, just so do they make wretched the mind that they have got hold of and seized, and hardly do they suffer it to get at the remedies which would cure it.
John CassianMore John Cassian quotes
The apostles were very sure that everything of theirs which had to do with salvation was a gift to them from God. 'Increase our faith'. They did not presume that the fullness of faith would come to them merely because they freely opted for it. They believed, rather, that it was a gift of God which would have to be granted to them.
John Cassian
... a Christian is quite certain to fall into the same sins which he condemns in another with merciless and inhuman severity, for 'a stern king will fall into misfortunes,' and 'one who stops his ears so as not to hear the weak, shall himself cry, and there shall be none to hear him' (Prov. 13:17; 21:13).
John Cassian