According to the Reformers, how does the doctrine of Total Depravity result in assurance of faith?

According to Luther and Calvin, a person has absolutely no ability to achieve salvation in himself/herself (due to total depravity) than by the sovereign will of God through unconditional election. It already assumes predestination, that those who are elected to be God's children will eventually be saved only because of His selection of you.

These quotes aren't directly from Calvin or Luther himself but reflect the gist of what it meant:

The Baptist Confession of Faith of 1689 says, "Those of mankind who are predestinated unto Life, God, before the foundation of the world was laid, according to His eternal and immutable Purpose, and the secret counsel and good pleasure of His will, hath chosen in Christ to everlasting glory, out of His mere free grace and love, without any other thing in the creature as a condition or cause moving Him thereunto."

Calvin's Institutes states, "All are not created on equal terms, but some are preordained to eternal life, others to eternal damnation; and, accordingly, as each has been created for one or other of these ends, we say that he has been predestinated to life or to death."

Therefore, one is saved from eternal separation and damnation from God not because they chose Him, but because - according to total depravity and unconditional election - humanity is unable to do so due to inherent wickedness. Hence, it is only in and through God - according to His sovereign will - that results in "...assurance of faith in the heart of the Christian."

Check out the doctrine of unconditional election here for more information as well. Hope it helps.

According to Reformers like Luther and Calvin, and modern reformed denominations, how does this doctrine result in assurance of faith in the heart of the Christian?

I could be wrong but my understanding of reformed theology would not connect these 2 doctrines in this way. Total Depravity simply speaks to the condition of mankind and his inability to save himself/herself. Assurance of Salvation is more connected to the doctrine of Eternal Security. Once a person is saved, they are secure in Christ by His righteousness being credited to their account. This person while saved, may waver for many reasons in their personal confidence in their assurance. Some reasons people waver include: sin, guilt, laziness, and bad doctrine.

To understand this more see the passage below. What causes these save people below to lose assurance of salvation?

2 Peter 1:5 Now for this very reason also, applying all diligence, in your faith supply moral excellence, and in your moral excellence, knowledge, 6 and in your knowledge, self-control, and in your self-control, perseverance, and in your perseverance, godliness, 7 and in your godliness, brotherly kindness, and in your brotherly kindness, love. 8 For if these qualities are yours and are increasing, they render you neither useless nor unfruitful in the true knowledge of our Lord Jesus Christ. 9 For he who lacks these qualities is blind or short-sighted, having forgotten his purification from his former sins.

Once I realise I am totally depraved then there is obviously going to be one place I am going to quit looking towards to try and get saved.... inwards. I am going to quit trying to do something meritorious to get my salvation. I am going to look away from myself to a much desired Saviour, and call out to Him to forgive me. And when he answers I will have assurance.

And what is more, the reflection that he saved me when I was so very bad, even at my worst, will encourage me to trust that I cannot lose my salvation because he took hold of me at my worst. Why then would he ever let me go?

Finally, as we go through life we see that we are much worse than we thought we were. If we already believe the doctrine of total depravity then such discoveries of ourselves will not overwhelm our faith... it will help us to be prepared for such self-discoveries, and to thank God that he knew about us all along.

It would seem that "total depravity" along with predestination is what defines Calvinism but I submit that all five points of Calvinism go hand in hand. Reformed theology emphasizes the doctrine of grace, best known by the acronym TULIP ,although this really doesn't correspond to the best possible names of the five doctrines.

Since "total depravity" is understood by most I will not take the time to go into great lengths here as you have given the basic meaning which is adequate for the question you have ask. We will need to jump all the way down to the "P" or the "perserverance of the saints" to get into the meat of your question. Actually, a better name for this doctrine would be "the perseverance of God with the saints." God perseveres with and for us, keeping us from falling away, which we would certainly do if He were not with us. Perseverance is the ultimate proof of our election.

The doctrine of perseverance does not rest on our ability to persevere, even if we are regenerated. Rather, it rest on the promises of God to preserve us. Paul writes to the Philippines,

"I am sure of this, He who began a good work in you will bring it to completion at the day of Jesus Christ" (Philippines 1:6)

It is by grace and grace alone that Christian's persevere. God always finishes what He has started. He assures us in His word that His purposes in election are not frustrated.

The golden chain of Romans 8 gives us even further testimony to this hope.

"And those whom he predestined he also called, and those whom he called he also justified, and those he justified he also glorified" (Romans 8:30) Paul goes on to declare that nothing "Will be able to separate us from the love of God in Christ Jesus our Lord" (8:39)

We have security and assurance because salvation is of the Lord and in His all knowing and craftsmanship He gives the Holy Spirit to every believer as a promise that He will fulfill what He has begun. He has sealed every believer by the Holy Spirit. Yes, He has marked us with an indelible mark and given us His personal down payment that guarantees he will finish the transaction.

A final basis of confidence is found in the high priestly work of our Lord Jesus, who interceded for us, just as He prayed for the restoration of Peter, He also prays for our restoration when we stumble and fall. We may fall for a season but we will never fully or finally fall away. His mighty hand and His promises we find in His Holy word will hold us up. Jesus prayed in the upper room,

while I was with them, I kept them in your name, which you have given me. I have guarded them, and not one of them has been lost except the son of destruction, that the Scripture might be fullfilled." (John 17:12)

Only Judas, who was the son of perdition from the beginning whose profession of faith was spurious was lost. Those who are true believers

cannot be snatched from God's hand (John 10:27-30.

As you can see, although we are a totally depraved people, God gives His chosen a new heart, a regenerated heart, a heart that accepts, cherishes and loves the very things that we were incapable of. He preserves us, He gives us assurance, He holds us up, He will not let His elect or chosen go even tho we are sinners from the very beginning..

5 point Calvints believe in the doctrine of the T-U-L-I-P not because of the belief in Calvin but the absolute truth of Gods word and His promises. His promises are for all that have come to saving faith alone given to us by His grace alone. The assurance that we have is not by OUR perseverance but by His preserving us just as He has promised thru-out the ages. We stand on His promises alone which gives us the confidence, even tho we were depraved from the beginning we are truly His now.

I hope this is a adequate answer to your question. At the heart of Calvinism, we are no different from any other true believer, we accept and believe all that God has revealed to us. We give all the glory and praise to a Sovereign God that deserves all of our worship. He alone is worthy. Thank you for your time.