2 Timothy 3:15 (Commentary Link)

For more on the expression, "holy writings" in Timothy, see http://www.studylight.org/commentary/2-timothy/3-15.html

As an example, from Robertson's WPNT:

The sacred writings (ιερα γραμματα — hiera grammata). “Sacred writings” or “Holy Scriptures.” Here alone in N.T., though in Josephus (Proem to Ant. 3; Apion 1, etc.) and in Philo. The adjective ιερος — hieros occurs in 1 Corinthians 9:13 of the temple worship, and γραμμα — gramma in contrast to πνευμα — pneuma in 2 Corinthians 3:6.; Romans 2:29 and in John 5:47 of Moses‘ writings, in Acts 28:21 of an epistle, in Galatians 6:11 of letters (characters). In Ephesus there were Επεσια γραμματα — Ephesia grammata that were βεβηλα — bebēla (Acts 19:19), not ιερα — hiera

However, Vincent's Word Studies has a slightly different take on the verse. He applies the terminology to "the learning acquired from Scripture by the rabbinic methods, according to which the Old Testament books were carefully searched for meanings hidden in each word and letter, and especially for messianic intimations."