Calvinism is simply the doctrine of determinism, that the future is determined. But since we are not told what that determined future is (we couldn't be told--- we would just do something else), this has no effect on the necessity of preaching or the quality of the preaching.

The Calvinist is just placing God outside space and time, so that the future is as visible as the past. But this doesn't mean that it is pointless to do stuff, because the future outcome is contingent on what happened. This is the old paradoxes of determinism, and they vanish with careful thinking.

This sort of teaching actually doesn't conflict with Calvinism. Ray Comfort's emphasis on the law and the penalty of sin as an evangelistic tool is not new to him; it has been employed by Calvinists for centuries. For example, Charles Spurgeon:

I say you have deprived the gospel of its ablest auxiliary when you have set aside the law. You have taken away from it the schoolmaster that is to bring men to Christ. No, it must stand, and stand in all its terrors, to drive men away from self-righteousness and constrain them to fly to Christ. They will never accept grace till they tremble before a just and holy law; therefore the law serves a most necessary and blessed purpose, and it must not be removed from its place.¹

John MacArthur similarly teaches that teaching of the law must precede teaching grace:

Grace means nothing to a person who does not know he is sinful and that such sinfulness means he is separated from God and damned. It is therefore pointless to preach grace until the impossible demands of the law and the reality of guilt before God are preached.²

The Parable of the Sower

This may seem counter-intuitive. To Calvinists, who believe in Unconditional Election, why would it matter how the Gospel is preached to sinners? Won't the elect be saved regardless? George Whitefield sheds some light on this subject:

But you may as well expect a crop of corn on unploughed ground, as a crop of grace, until the soul is convinced of its being undone without a Saviour. That is the reason we have so many mushroom converts, so many persons that are always happy! happy! happy! and never were miserable. Why? Because their stony ground is not ploughed up; they have not got a conviction of the law; they are stony-ground hearers.³

Whitefield, of course, is referring to the Parable of the Sower, which describes four types of people: the unresponsive, the impulsive, the preoccupied, and the well-prepared. A more extensive treatment of this passage from a Calvinist perspective is available, but for now, we notice that Whitefield is arguing that many of the "converts" he sees, because they never understood the law, are actually not Christians: the seed fell on rocky soil, and so it did not grow.

Other Calvinists draw similar lessons from this parable. The people whose hearts are like rocky soil have emotions that are "superficial, not based on deep-seated convictions," according to Hendriksen.⁴ Calvin writes that "their hearts are not so properly and thoroughly subdued, as to have the softness necessary for their continued nourishment."⁵

Calvinism therefore has no objection to "plowing the soil" and preaching the law prior to preaching grace. The condition of the sinner's heart when he expresses faith matters.

But what about election?

It's true that there is mystery in the doctrine of election. But just because we don't fully understand how evangelism and election work together does not mean, to Calvinists, that it is acceptable to preach an incomplete Gospel or not preach at all. Even accepting that our faulty methods will not keep anyone out of heaven, there are two very good reasons to preach the law to those with "rocky" hearts:

• Some, being taught the law, would reject Christianity rather than be falsely converted. This is preferable to having a false hope of salvation (cf. Revelation 3:15–17; Matthew 7:21–23).
• Others, being taught the law, will have their hearts softened and will be truly converted. Without the teaching of the law, God's plan might have been that they be converted later in life.

Ultimately, it comes down to this: given that we do not know who the elect are or when they will come to Christ, it is the evangelist's responsibility to preach the Gospel faithfully (Matthew 28:18–20). Many Calvinists since the Reformation have regarded the teaching of the law as a critical component of such a faithful Gospel witness.

Summary

Ray Comfort's emphasis on the law, far from contradicting Calvinism, aligns nicely with it. Faith in the certainty of God's election does not give the Calvinist an excuse to fail to faithfully preach the Gospel, and many Calvinists find the law to be an essential part of it.

References:

1. "The Perpetuity of the Law of God," May 21, 1882. (Google Books)
2. Galatians, MacArthur New Testament Commentary, page 93.
3. "The Gospel, A Dying Saint's Triumph," in The Revived Puritan, 422.
4. Hendriksen, Exposition of the Gospel According to Matthew, 561.
5. Calvin, Commentary on Matthew, Mark, and Luke, Volume 2.

^{Note: All emphasis in quotes has been added}