Hebrews 6:4-6 warns of the impossibility of renewing those who fall away after tasting grace — parallel to the danger of wasted suffering.

Hebrews 10:32-39 exhorts endurance and warns against shrinking back, echoing the fear that suffering might be in vain.

2 John 1:8 urges watchfulness to not lose reward — directly parallel to Paul's concern that their suffering might be in vain.

Luke 8:13 illustrates temporary faith that falls away in testing – exactly the 'in vain' Paul warns the Galatians of.

1 Corinthians 15:2 uses the identical phrase 'believed in vain' to warn that saving faith must be held fast.

2 Corinthians 6:1 warns against receiving God's grace in vain – echoing Paul's concern that their sufferings may be wasted.