This profound confidence comes at the conclusion of a threefold summary of Paul's experience while serving the Lord Jesus, as well as his deep bond with the church at Philippi. The "salvation" of which Paul speaks references victory in this life as much as the eternal rescue at the end.
Initially, the confidence comes "through your prayer, and the supply of the Spirit of Jesus Christ" (Philippians 1:19). Christian leaders covet the prayers of those with whom they serve. Twice Paul specifically asked the Thessalonian church to pray for him (1 Thessalonians 5:25 and 2 Thessalonians 3:1). Most of us are familiar with the promise that the "effectual fervent prayer of a righteous man availeth much" (James 5:16). Should we be surprised when such prayer brings boldness?
There is also the experience that the "supply" of the Spirit of God engenders faith. God's providential care, experienced during the testing of our life, produces a growing hope and confidence (Romans 5:4-5). It is most often true that we learn more of God's faithfulness in times of need than in times of plenty.
Then there is an "earnest expectation and . . . hope" that result in "boldness" (Philippians 1:20). Growing confidence in the Lord's provision and protection undergirds an anticipation for God's direction and wisdom. Expecting something to happen is the flip side of hope. Experience in kingdom work brings spiritual joy and peace (Romans 15:13).
Thus, the confident statement "now also Christ shall be magnified in my body, whether it be by life, or by death. For to me to live is Christ, and to die is gain" (Philippians 1:20-21). HMM III http://www.icr.org/article/5968/