The apostle Paul had two primary burdens in life: the first coming of Christ and the second coming of Christ. The first was about bearing sin (cf. Rom. 5:8; 2 Cor. 5:21; Gal. 3:13), while the second was about bringing salvation (cf. Rom. 8:23; 1 Cor. 15:23; Phil. 3:21). This is what consumed the man and “compelled” him in ministry (cf. 2 Cor. 5:14). However, it takes the highest level of faith to believe in the reality of these two comings.
On the one hand the spirit of the age militates against belief in the second coming and the Day of the Lord. God won’t really fix everything; Jesus won’t really return with blazing fire and angels to judge the living and the dead; we won’t really be held to account for every word and deed; etc. It takes faith to believe that Jesus is real, that he is alive, and that God “has set a day when he will judge the world with justice by the man he has appointed.” (Acts 17:31) The reality of the Day of the Lord is woefully neglected in the preaching and teaching of the modern church.
However, even more difficult to believe is that on that day the only thing God will account as righteousness is the sacrificial atonement of Jesus Christ (cf. Rom. 3:24f; 5:9f; Eph. 2:12f; Col. 1:21f; Tit. 3:7). It takes an even higher degree of faith to believe that when we stand before him at his second coming, the only means of escaping the wrath of God and inheriting eternal life is by faith at the heart level in the Sacrifice on our behalf. Every other approach to God will be deemed “a righteousness of my own” (Phil 3:9; cf. Rom. 10:3), i.e. self-righteousness, which will promptly inherit a Lake of Fire. This is counter-intuitive at every level and commonly causes sincere believers, like the apostle Peter(!) and Barnabas (Gal. 2:12f), to “set aside the grace of God” (Gal. 2:21), become “severed from Christ” (Gal. 5:4), and thus “live as enemies of the cross of Christ” (Phil. 3:18). Of course, they genuinely believe themselves to be good with God, but they have in fact “lost connection with the Head” (Col. 2:19), and lamentably “their end is destruction” (Phil. 3:19).
If the world rejects the reality of the Day of the Lord, then the religious reject the reality of the Cross—and both live according to the flesh and reject the Spirit of God (cf. Rom. 8:3f; Gal. 3:3; Phil. 3:3). Thus, the challenge of faith is to “press on” in justification by faith “toward the goal” of the return of Jesus and the resurrection (Phil. 3:14). The life we live in this age we live by faith in the Son of God, who loved us and gave himself for us (Gal. 2:20). This is called “running a good race” (Gal. 5:7), i.e. “a good work”, which God himself will carry on and “bring it to completion at the day of Jesus Christ” (Phil. 1:6).