Harden not your heart
From Hebrews 3
12 Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.
13 But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
14 For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
15 While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.