The full value of Christ’s gift (which is the gift of salvation) is expensive and thus, shouldn’t be taken for granted.
Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling; Philippians 2:12.
Salvation is not something we just receive and then take for granted like winning the lottery. This is because winning a large sum of money you don’t know how to use it. If you are to experience the joy and benefit that God has in store for you, the gift of salvation should not only be received but also used well – with fear and trembling (Adapted).