Question:
Why did Paul tell Timothy that the holy Scriptures would make him “wise unto salvation through faith which is in Christ Jesus” (2 Timothy 3:15) when Paul says in 2 Timothy 1:5 that he already had faith in him?

Answer:

Paul is encouraging Timothy to stay in the Word of God, because it is through the Word that Timothy has his faith in Christ. “So then faith cometh by hearing, and hearing by the Word of God” (Romans 10:17).
There are three aspects of salvation: “(1) Who delivered us from so great a death, (2) and doth deliver: (3) in whom we trust that He will yet deliver us” (2 Corinthians 1:10).
(1) Salvation from the PENALTY of sin, which we receive the moment we confess that we are sinners, and trust in Christ.
(2) Salvation from the POWER of sin in our lives after we are saved.
(3) Salvation from the PRESENCE of sin when we are taken home to glory.

(1)  Timothy had salvation from the penalty of his sins from the moment he placed his faith in Christ and in the work He did for Him on the cross of Calvary (2 Timothy 1:5).

(2)  But he needed salvation from the power of sin (2 Timothy 3:15), which we receive by being occupied with Christ when we meditate on the holy Scriptures and trust His strength to please Him.

(3)  The Lord desires that we “watch” for His coming: “What I say unto you I say unto all, Watch” (Mark 13:37). “Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when He shall appear, we shall be like Him; for we shall see Him as He is.”

The benefit to us for watching for His return, is: “And every man that hath this hope in Him purifieth himself, even as He is pure” (1 John 3:2, 3).