2 Timothy 3:15
From a child = Timothy could not recall a time in his life when he had not known the Scriptures. Jewish parents had the duty their children the law when in their fifty year.
The term "sacred writings" or "holy writings" here refers to the Old Testment. Paul will use a different term in the next verse to include the inspired writings of men in the New Testament age as well.
A good deal of the NT scriptures had already been written by this time and were already being circulated ( seeCol_4:16).
Note the formular for introducing a "thus saith the Lord" .. Ephesians 4:8; Ephesians 5:14; 1 Thessalonians 4:15
To make wise = to impart wisdom
Wise unto salvation ..= The OT pointed out and identified the Messiah when he came into the world.
Faith .. in Christ Jesus = faithful adherence to the teachings of Chist, true fidelity, and in Him.