“woe.” The Greek word is ouai (#3759 οὐαί; pronounced ooh-eye’). For an explanation of the meaning of “woe,” see commentary on Matthew 11:21. In this context, “woe” is an expression of warning of the grief and disaster that is coming to the experts in the Law and Pharisees if they do not repent and change their ways.
“faithfulness.” “Faithfulness” is a much better translation of the Greek word pistis (#4102 πίστις), in this context than “trust.” Faithfulness to God and the covenant Israel made with God was one of the most important things in the Law. God was a “faithful” God (Deut. 7:9; 32:4), and God’s people were to be faithful to Him (Ps. 31:23; 101:6; Isa. 1:21, 26). Indeed, faithfulness is a fruit of the spirit (Gal. 5:22). But the religious leaders at the time of Christ were not being faithful to God or to the Law and covenant, and Jesus pointed that out to them in clear language.