The phrase Truly I tell you often introduces the sayings of Jesus and emphasizes their importance (see also 8:12; 9:1, 41; 10:15, 29; 11:23; 12:43; 13:30; 14:9, 18, 25, 30).
the sons of men will be forgiven all sins and blasphemies: One sin has eternal consequences. This is blasphemy against the Holy Spirit. To blaspheme the Holy Spirit means attributing the work of God to an unclean spirit (an evil spirit). Resisting and rejecting God's work in this way prevents the convicting work of the Spirit, which leads to repentance, faith in God, and forgiveness of sin (see study notes on Matthew 12:31–32; compare Hebrews 6:4–6; 1 John 5:16–17).