The letter was from Paul and his faith and how he compares it to Timothy's faith as he describes how his (Pauls) faith in Jesus sustained him through what he describes in the preceding verses.
10 But thou hast fully known my doctrine, manner of life, purpose, faith, longsuffering, charity, patience,
11 Persecutions, afflictions, which came
unto me at Antioch, at Iconium, at Lystra; what
persecutions I endured: but out of
them all the Lord delivered me.
12 Yea, and
all that will live godly in Christ Jesus shall suffer persecution.
13 But evil men and seducers shall wax worse and worse, deceiving, and being deceived.
14 But continue thou in the things which thou hast learned and hast been assured of, knowing of whom thou hast learned
them;
15 And that from a child thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus.