A resting in notions of Gospel truth, or in the recollection of past comforts, without a continual thirst for fresh communications from the Fountain of life, is, I am afraid, the canker which eats away the beauty and fruitfulness of many professors in the present day; and which, if it does not prove them to be absolutely dead, is at least a sufficient evidence that they are lamentably sick. -John Newton to John Ryland
In other words to rest upon a past experience or a mere belief in certain Christian doctrines without having a desire for a fresh drink from the Fountain of Life is a sure sign that you are either spiritually dead or at least very sick.
Newton can say this because Christ is exceedingly beautiful. One does not have a true experience of the beauty and majesty of Christ and then forever turn away without longing for more of Jesus. To do so either means that you have never actually beheld the glory of God or that something has gone so awry in your heart that you have become like a man dying of thirst that rejects water because he craves salt instead.
What do I do if this describes me?
Repent and turn to Jesus: either as a redeemed but sin-sick believer or as an unbeliever that needs to see Jesus for the first time.