28. There have been times when 1 had very definite assurance of my salvation, and then I have lost it again. Why do these periods of darkness come?

There may be various reasons for these periods of darkness. The greatest saints have at times known the same experiences. They may possibly be accounted for by great mental weariness and physical weakness. The adversary of our souls is always ready to take advantage of such conditions, and ever seeks to make us forget the clear, definite promises of God on which we have rested when well and strong.
There is an authentic story told of an aged minister, who had preached the gospel in clearness and power during all his public life, but who, when he was suffering at times, found himself greatly beset by doubt and uncertainty. Mentioning the matter to his wife, she drew his attention to John 5:24. As he read the precious words again, "Verily, verily, say unto you, He that heareth my word, and believeth on him that sent me, hash everlasting life, and shall not come into condemnation; but is passed from death unto life," he burst into a joyous laugh, and said, "How strange that I should ever forget words like these, when I have preached on them myself for years.”
Sometime later the wife came into the room and found her aged husband leaning over the side of the bed, holding the open Bible beneath his couch. She exclaimed, "Whatever are you doing?" He answered, "Satan has been after me again and as he is the prince of darkness, I took it that he would be in the darkest place in the room, which is under the bed, and so I was just showing him John 5:24, and the moment he saw it he ceased to trouble me.”
We can quite understand the mental weakness that the story suggests, but the principle is blessedly true. When the adversary of your soul comes against you seeking to destroy your confidence, show him what God has said.
But there may be other reasons which account for the loss of that blessed assurance you once enjoyed. The apostle Peter suggests such in his Second Epistle, chapter 1, verse 9. In the previous verses he has been stressing the importance of spiritual growth, and the believer is instructed to be diligent in adding to his faith virtue, and to virtue knowledge, and to knowledge self-control, and to self-control patience, and to patience godliness, and to godliness brotherly kindness, and to brotherly kindness love; and then he can be sure that if these things are in him and abound, he will not be idle nor unfruitful in the knowledge of our Lord Jesus Christ.
But, on the other hand, if the believer is neglectful of these things, he cannot expect the divine blessing to rest upon him; and so we are told, "He that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins." There is something very solemn here. Notice, he was purged from his old sins, but through indolence and carelessness he has lost the assurance of this. The blessedness of by-gone days has faded from his memory.
The Christian life is never static. One must either grow in grace, or there will be backsliding and deterioration. "The backslider in heart shall be filled with his own ways" (Prov. 14:14). He who does not go on with God, but allows himself to drift, is almost sure to lose the joy of his salvation. Examine yourself as to this matter, and if you find that you have been careless in regard to the study of your Bible, careless as to your prayer life, careless as to the proper use of the means of grace, confess all this to God and give diligence to walk with Him in days to come, that you may develop a stronger Christian character.
Last of all, let me remind you that any known sin condoned in your life will rob, you of the joy and assurance of your salvation. "If I regard iniquity in my heart, the LORD will not hear me." Many a one who has gone on happily with Christ for sometime, but through toying with sin has become ensnared and entrapped into something that has so grieved the Spirit of God that he has lost his sense of acceptance in Christ. See to it that there is no unconfessed sin in your life. Be sure that you are not tolerating any secret sin which is draining you of spiritual power and hindering your communion with God.
Worldliness, carnal indulgence of any kind, unfaithfulness as to your Christian responsibilities, unseemly levity, the harboring of malice or ill-will toward others—all or any of these things are calculated to destroy your sense of assurance. If guilty of any of them, face things honestly in the presence of God, remembering that He has said, "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.”
Do not accept the suggestion of the tempter that you are powerless to break away from evil habits. Remember it is not a question of your own power, but when you honestly repent of the wrong-doing and turn to the Lord for divine help to overcome your besetting sin, He will undertake for you. As you reckon yourself to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord, the Holy Spirit will work in and through you, causing you to triumph over tendencies toward evil, and enabling you to live victoriously to the glory of the God who has saved you.