‘For I know that this will turn out to my salvation, through your supplication and the supply of the Spirit of Jesus Christ,'
One thing that Paul was certain of was that God was in control, and that his imprisonment would bring ‘salvation' one way or the other. Either because through death he would enter into the fruits of God's salvation in Christ, or because through being set free he would experience salvation and deliverance, both in body and in spirit, and be among them again all the better for his experience.
And this would happen firstly because the Philippians and others were praying for him, and upholding him, and secondly because the Spirit of Jesus Christ was active in his case. It was He Who would guide him in his defence as Jesus had promised, and either allow his execution, or arrange for him to be set free (e.g. Matthew 10:19-20). Furthermore he knew that He would strengthen him to meet whatever situation faced him. The verb ‘supply' has within it the idea of undergirding and strengthening. He knew that he was undergirded and strengthened by the Spirit of God. How then could he be afraid?
‘My salvation.' Scripture portrays ‘salvation' from a number of angles. Sometimes it is seen as a once for all thing, guaranteed from start to finish from the moment of believing, as the sole work of Jesus Christ the Saviour (2 Timothy 1:9; Titus 3:5); sometimes as something that had happened and was now having present effects (Ephesians 2:5; Ephesians 2:8-9); sometimes as a present process continually going on (1 Corinthians 1:18; 2 Corinthians 2:15; Philippians 2:12; 1 Timothy 2:15); and sometimes as a future prospect when it would be brought to completion (Philippians 1:6; Php 1:28; 1 Corinthians 3:15; 1 Corinthians 5:5; 2Co 7:10; 1 Thessalonians 5:9; 2 Thessalonians 2:13).
We are not, however, to see him here as simply concerned with receiving benefit from his own personal salvation, but rather as wanting his ‘salvation' to be a vindication of God's word and ways. It is indicating that he was concerned with the fact that that salvation would be a vindication of his ministry and message proclaimed by God Himself. Whether released to continue on in the service of Christ, or whether taken through death to a higher privilege of heavenly service in the presence of Christ, he was confident that his message and stand for Christ would be vindicated and thus be a firm witness. For this use of ‘salvation' compare Job 13:16 in LXX where the Greek wording is identical with Paul's (and may well have been in Paul's mind), ‘though He slay me, yet I will wait for Him, nevertheless I will maintain my ways before Him, this also will be my salvation, for a godless man shall not come before Him'. In other words Job was convinced that whether in life or in death he would be saved and vindicated as a result of his acceptance before God. The same was now true for Paul. If he died and came before God this would be evidence of the genuineness of his salvation, and would vindicate all that he had proclaimed and stood for (compare Philippians 2:16-17). If he lived on it would indicate God's protecting hand upon him, and thus vindicate his message.
Note the combination of man's prevailing through prayer and God's sovereignty through the Spirit. It is not a matter of either/or but of both/and. God is sovereign, but it is as we pray and cooperate with God in His work, that God carries out His sovereign purposes.