Saint Seraphim directs us to the following Psalm,
Upon his mind there must always be engraved these words of the prophet: "Serve the Lord with fear, and rejoice with trembling" (Ps 2:11)
All our actions must be done with this in mind. It is only in this way that our works will aid us in our aim to be united with Him. Saint Seraphim points out that without such an attitude, instead of being blessed, we will be cursed. "Cursed be he that doeth the work of the Lord negligently" (Jer. 48:10).
Our work is great and difficult. We need to always cooperate with God's grace.
Saint Seraphim says,
Reverent carefulness is necessary here because this sea--that is, the heart, with it's thoughts and desires, which one must cleanse by means of mindfulness-- is great and vast, "and there are numberless reptiles there" (Ps 103:27), that is, numerous vain, unjust, and impure thoughts generated by evil spirits.
Much more than faith is necessary. In cooperation with divine grace, we have to do the work necessary to tame the impulses of our biological being, so that all our actions can be directed according to God's will and not be based on our own desires and fears because of our mortality and susceptibility to sickness and suffering.
Seraphim of Sarov,
Reference: Little Russian Philikolia, p 27