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 CassianMore John Cassian quotes
... 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
It is not our free will but 'it is the Lord who sets the captive free' (Ps. 145:7). It is not our own virtue but 'it is the Lord who lifts up those who were laid low' (Ps. 145:8). It is not application to reading but 'it is the Lord who gives light to the blind' (Ps. 145:8). It is not our cautiousness but 'it is the Lord who protects the stranger' (Ps. 145:9). It is not our endurance but 'it is the Lord who raises or gives support to the fallen' (Ps. 144:14).
John Cassian