Question: I have struggled with the “assurance” of salvation for years and have never
had the peace or understanding of what it means. Is there a line in the sand
in which we can cross and have that assurance? I am continually questioning
whether or not I am saved, and I have anxiety about it at times, because it
seems to me that if I sneeze wrong I am lost. We all are sinners so where
does this assurance of salvation fit in? And can I have peace about it
without the constant agonizing question?

Answer: I can certainly relate to you question about assurance and the agony. My own journey for assurance began with whether or not such a thing as assurance of salvation was even biblical—something I had been led to question at one point. Answering this question then led to a clear understanding of why we wrestle with the idea—the devil does not want us to have assurance of our salvation in Christ.

The clearest answer for both of these ideas is found in Christ’s wilderness temptation when the devil questioned three times whether Christ was the Son of God (assurance). And he tried to get Christ to base His response on something (turning stone to bread/works) other than the word of God (This is my beloved Son in whom I am well pleased).
No true believer wants to have their faith undermined by lacking assurance of their salvation. There is almost nothing as disheartening as not knowing whether or not the believer has salvation in Christ. This is why the Bible affirms both powerfully and positively the absolute surety of the believer’s salvation in Christ. 
In 1 Thessalonians 1:5 God encourages us that there is assurance we can have of our salvation in Jesus Christ. “For our gospel came not unto you in word only, but also in power, and in the Holy Ghost, and in much assurance; as ye know what manner of men we were among you for your sake.” Assurance is a part of the new covenant experience spoken of in Hebrews 10:22, 23: “Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water. Let us hold fast the profession of our faith without wavering; (for He is faithful who hath promised).”
God is encouraging us to have assurance because “He is faithful.” God is giving us much assurance, yes; He is giving us full assurance, for He is faithful who has promised. And we need to be encouraged with the faithful promises of God.
“And this is the record that God hath given to us eternal life, and this life is in His Son. He that hath the Son hath life and he that hath not the Son of God hath not life.  These things I have written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God” (1 John 5:11-13).
The reason that God has given us assurance is so that we may not only know that we have eternal life because we believe in Jesus but also that we may believe or continue to believe on the Son of God.
We can be assured that He will finish the work He has begun in us. Look at Philippians 1: 6, “Being confident of this very thing that he which hath begun a good work in you will perform it until the day of Jesus Christ.” Can we be confident of that? Can we have assurance in that? Indeed we can. In fact, the word “assurance” in 1 Thessalonians 1:5 means “entire confidence” in the Greek.
This is to be the effect of righteousness, even the righteousness of Jesus Christ. “And the work of righteousness shall be peace; and the effect of righteousness quietness and assurance forever” (Isaiah 32:15, 17). God wants to instill assurance within our hearts so that we may continue to believe in Jesus Christ. He doesn’t want us going about doubting and wondering in darkness and unbelief.  He knows discouragement will press upon us, for he was once like us upon this earth as a man compassed about with temptations. He was “in all points tempted like as we are” (Hebrews 4:15).
The devil came and said, “If thou be the Son of God…” and Jesus went back to the Word: “It is written.” The devil came again and put him upon the pinnacle of the temple and he said “If thou be the Son of God…” The devil was pressing upon him the idea that he was not the Son of God. Have you ever experienced that? You’re not a son or daughter of God! Look at you, look at your weaknesses. Look at your cravings and desires! Look at the bents, the inclination of your heart. You don’t want to remain faithful to the word of God, your whole being craves these things that are out of harmony with God’s word. Isn’t that what Satan was saying to Jesus? You’re not the Son of God! In what could Christ trust? Where could He find the promised security of acceptance with God? 
Look back to Matthew 3:16 and 17. As Jesus was baptized and the spirit of God came upon Him, a voice from heaven said “This is my beloved Son, in whom I am well pleased.” Here is Christ’s assurance. Here is given the “much assurance” that He was the Son of God. God spoke from heaven and he said, You are my Son. This assurance was given to Christ so that he could continue to believe that he was the Son of God through trial and temptation. Do we have trial and temptation?  Do you think God wants to strengthen and encourage us? Indeed He does. In fact you are going to find over and over again throughout the life of Christ as he was compassed about with temptations and with trials and darkness, God encouraged him with assurance of His love and acceptance. Christ went through the same struggle that you and I go through with assurance—but He conquered by faith in the Word not in Himself, His works or His circumstances.
Consider also Matthew 17:5, “While He yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye Him.” Christ was nearing the close of His ministry. He was about to endure some of His greatest trials. “Now is my soul troubled”— have you ever struggled, have you ever found it difficult to press through the darkness? Jesus knows what that feels like, His soul was troubled— “And what shall I say? Father, save me from this hour: but for this cause came I unto this hour. Father, glorify thy name. Then came there a voice from heaven, saying, I have both glorified it, and I will glorify it again” (John 12:27, 28).
Jesus Christ needed assurance and so do we. Don’t we? God wants to encourage us with much assurance. He wants to encourage us that He started the work and He’ll finish the work. He wants us to understand that when we are compassed with darkness and trial; when we see the depravity of our own soul and the desire we have for those things that are of the world, He can and will rescue us. He wants to assure us that we are his children and that we can have hope in Him. Praise God for His blessed assurance