1 Corinthians 15:1-2
1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
Abridged Commentary - 1 Corinthians 15:1-2
15:1-2 The "salvation" (sōzō) spoken of in this passage refers to sanctification and deliverance brought about by abiding in sound doctrine (1 Timothy 4:16). The disciples would be sanctified by their abiding in the faith, unless what they believed was powerless and false - "if Christ is not risen, then is our preaching vain, and your faith is also vain" (1 Corinthians 15:14, 15:17). "Believing in vain", then, refers to the object of one's faith.
The Grace New Testament Commentary - 1 Corinthians 15:1-2
15:1-2 Paul declares (lit., "makes known") to the believers the gospel which [he] preached to them. When it was first preached to them, they received the gospel message. In addition to bringing justification, it is the gospel in which [believers] stand - the perfect tense suggests continuing results. Christians therefore stand in Christ because of the gospel (cf. Eph 1:3-14). Furthermore it is the gospel by which also [believers] are saved, if [they] hold fast that word.
This statement stresses the fact that the gospel includes more than justification, which brings forth eternal life; it also includes a daily sanctification (are saved) if believers hold fast (or abide in) the word (cf. John 8:31-32; Rom 1:15; 10:9: 1 Cor 15:2; Gal 2:20; Eph 2:10; Jas 1:21). This daily sanctification process relates to the quality of life the Christian will spend in eternity (1 Corinthians 3:9-15; Luke 19:11-27; Rom 8:16-17: 2 Pet 1:10-11). However, if the resurrection of a physical body could not be verified, belief in the gospel would be in vain - having no purpose (cf. 1 Cor 15:12).