Faith is not something we earn; it is something that we receive. It is a gift
"I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me." (Galatians 2:20)
The faith that a believer in Christ lives by, is not even the believer's faith! When we come to Christ, we are so broken, so lost, so empty, all we can give to God is ourselves, our sinful selves, which he takes and transforms into the righteousness of God (cf 2 Corinthians 5:21) Through Christ, God reconciles us to Himself and imparts to us "the faith of the Son of God" — His Son's faith resides in the believer.
Consider also the full context of Paul's letter to the Ephesians:
"For by grace are ye saved through faith; and that not of yourselves: it is the gift of God" (v. 8) The saving faith that brings a man to recognize and receive the grace imparted to us by Christ's death on the Cross, this saving faith is a gift from God!
And so that no one would foolishly assume that they had anything to do with earning the saving grace of God, Paul continues:
"Not of works, lest any man should boast. For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them. (Ephesians 2: 9-10)
Ever believe does not exercise an active role in his salvation, having received it and the means to achieving it both as a gift from Him. Instead, God himself places the works that every believer is fitted to do within him, established even before the believer was saved.
The passage Ephesians 2:8-10 verse does not support the theological principle of predestination, yet it certainly underscores that ever person who receives Jesus Christ as Lord and Savior does so independent of any work on his or her part. Rather than leaving a believer in despair as to whether he may or may not be saved, this is a great relief, for God has already done the work of reconciling us to Himself through His Son (cf 2 Corinthians 5:18)
So, if faith is a gift, one which God has already accorded to us, then what about those who feel that they have lost their faith? What is really going on when a believers claims to be enduring "The Dark Night of the Soul"
When a believer tries to get to God by his own efforts, whether by making long prayers, fasting extensively, or doing good works, he falls from grace, he frustrates the very grace which God has already given to him. Imagine the very unreasonable nature of trying to work for something which has already been prepared and delivered as a gift, unconnected with any effort or merit on the part of the recipient.
For believers who rely on their own efforts, they will not be able to "get to God" or feel his presence. Praying for something that a believer already possesses will inevitably make him feel as though God is not answering his prayers. Because he does not sense His presence, he begins to think that God has abandoned him.
In reality, God is faithful to his promise: "For he hath said, I will never leave thee, nor forsake thee" (Hebrews 13:5). We cannot even leave him! Yet if we do not believe, we end up striving for something that we already have by faith! For God, faith is everything:
"But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him." (Hebrews 11:6)
Yet seeking God is not a matter of striving, but a matter of receiving and believing on Him:
"Jesus answered and said unto them, This is the work of God, that ye believe on him whom he hath sent." (John 6:29)
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To recap, "a loss of faith" is actually a calling for a believer to walk more by faith, not by sight (cf 2 Corinthians 5:7) It is important to stress for believers that the Holy Spirit who moves among us is more real than the world we sense and discern with our minds. By faith we claim what is within our spirit man, which in turn daily renews our minds to discern spiritual things. Our bodies may grow old, but even our spirit man is renewed daily.
A Dark Night of the Soul is a prompting for a believer to live more by faith, trusting Him to do everything in the believer, and for the believer to believe and receive His Grace and Goodness without trying to achieve it for himself.