Acts 24:27 When two years had elapsed, Felix was succeeded by Porcius Festus. And desiring to do the Jews a favor, Felix left Paul in prison.
Acts 23:11 The following night the Lord stood by him and said, “Take courage, for as you have testified to the facts about me in Jerusalem, so you must testify also in Rome.”
Paul remained in prison for at least two years waiting to be sent to Rome. We have no scriptures indicating what Paul went through while in that prison. Some will wonder, some will not, because doubt is a human condition that many of us suffer.
What Paul went through during those years is not the point because what we have in scriptures is what Paul did afterwards. His attitude and actions when he was brought before King Aggrippa might lead us to believe that nothing bad happened to Paul during that time.
Study history of the situations during those times, it is likely that the experience was harsh even if he was only kept in isolation. When we are left without evidence we begin to have doubts. Doubt is natural, of the flesh, not of God.
Here is an opinion of mine and not to be read as evidence of faith. Paul didn’t have a bible of any type to comfort him in prison. He had Jesus.
John 14:18 I will not leave you as orphans; I will come to you.
While Jesus said this to His apostles, Paul is an apostle called by God, so I expect that promise applies.
Those thoughts comfort me about Paul. When doubts surround me, I try to remember I too am chosen. Not as an apostle but I look first to what responsibilities I have been given and not to allow doubt to interfere. Here is something I ponder.
1 Corinthians 12:12-20 English Standard Version
12 For just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. 13 For in one Spirit we were all baptized into one body—Jews or Greeks, slaves or free—and all were made to drink of one Spirit. 14 For the body does not consist of one member but of many. 15 If the foot should say, “Because I am not a hand, I do not belong to the body,” that would not make it any less a part of the body. 16 And if the ear should say, “Because I am not an eye, I do not belong to the body,” that would not make it any less a part of the body. 17 If the whole body were an eye, where would be the sense of hearing? If the whole body were an ear, where would be the sense of smell? 18 But as it is, God arranged the members in the body, each one of them, as he chose. 19 If all were a single member, where would the body be? 20 As it is, there are many parts, yet one body.