Jamieson, Fausset and Brown on 2 Timothy 3:15

15. from a child--literally, "from an infant." The tender age of the first dawn of reason is that wherein the most lasting impressions of faith may be made.

holy scriptures--The Old Testament taught by his Jewish mother. An undesigned coincidence with 2Ti 1:5; Ac 16:1-3.

able--in themselves: though through men's own fault they often do not in fact make men savingly alive.

wise unto salvation--that is, wise unto the attainment of salvation. Contrast "folly" (2Ti 3:9). Wise also in extending it to others.

through faith--as the instrument of this wisdom. Each knows divine things only as far as his own experience in himself extends. He who has not faith, has not wisdom or salvation.

which is in--that is, rests on Christ Jesus.