When Paul addressed his epistle / letter to Timothy, he wrote by the authority of God. This means, he did not write his opinion, but he wrote that which the Lord expects those who read to hear and heed.
How can one be sure that he wrote by the authority of the Lord and not just his opinion? First, consider that he identified himself as an apostle of the Lord by the command of God (1:1), one who was guided into all truth, Jesus said (John 16:13). Second, he wrote to Timothy about a command he entrusted to him (1:18). Third, he made it clear the purpose and authority under-girding his writing (3:14-15). Fourth, he said to Timothy, "These things command and teach" (4:11, ASV). Fifth, he understood what he wrote to be the authority of God (6:3), something similar to what he wrote in Galatians 1:6-9.
Why is all this important? Because there are some well-meaning and misguided people in the religious world who say Paul wrote only for his time, in matters related to his culture. It is certainly the case that Paul addressed his words in a specific culture, but God's word is not exclusive to a culture of the first century. It applies to the many and all cultures since that time; it applies today.
We have all things that pertain to life and godliness through Him that called us. As it was then with this message, it is the same today with the exact same message. No alterations, no additions, no subtractions.