What is the evidence of salvation?
The evidence of salvation is receiving the Holy Spirit, obtained by the hearing of faith rather than by works of the law. Salvation is faith plus nothing, resting on the finished work of Christ; it is not earned by keeping the law.
Les explains that Paul insists salvation is faith plus nothing, and if it were otherwise, Christ died in vain. Paul asks whether they received the Spirit—the evidence of their salvation—by the works of the law or by the hearing of faith. Les then cross-references Romans 10:17, where faith comes by hearing, and hearing involves no work. The evidence is the Holy Spirit received through faith, not law-keeping.
Galatians 3:2
This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith?