I've been told that it's rare to find someone who combines Dispensationalism and Reformed/Calvinistic doctrine (such as John MacArthur). Why is this? What ideas don't mesh well?
The reason these two are seldom found together seems to be primarily because of the difference of the view of the church between covenant theology and dispensationalism. Although some (like MacArthur) continue to maintain the distinction between the church and Israel (as in dispensationalism), most 5 point Calvinists follow covenant theology which denies this distinction - placing the beginning of the church at least back to Abraham, and logically back to Adam. Therefore, although dispensationalism maintains that everyone has always been saved by grace through faith, the specific object of that faith in Reformed theology is understood to be Christ - while dispensationalism would recognize the role of the progress of revelation such that the basis for salvation was the Cross, the object was not as clearly seen as the incarnate Son of God. Actually, it is even more complex than this, but this gets you going in the right direction.
As an ex-Calvinist turned mid-Acts dispensationalist, I find there are two primary points of contention that lead to all the rest:
Covenant/Reformed theology's tendency to allegorize or spiritualize various parts of Scripture vs dispensationalism's tendency to take it literally where it seems literally intended.
Covenant/Reformed folk (and others, to varying degrees) general belief that God is forever done with national Israel, having replaced her with the Body of Christ which has inherited the covenants and promises, and even some of warnings, God gave to Israel. Dispensationalists, on the other hand, believe God is not finished with Israel but has only set her aside in blindness temporarily during this dispensation of grace (Romans 9-11). Once this dispensation ends, God will once again take up dealing with Israel as His covenant nation, kicking off the apocalyptic end times events.
Point #1 alone is enough to keep the two systems from EVER synchronizing or harmonizing. Sad to say, it basically results in reading two different Bibles.
One reason could be that Reformed doctrine takes covenantal thinking into account; this allows for quite a bit of continuity between the two Testaments, with also some very distinct discontinuities. Reformed doctrine, for example, would see the covenants in the OT as building, one upon the other, in scope, until you have the New Covenant with Jesus Christ, which is so powerful it's retroactive in including the OT saints. Dispensationalism, on the other hand, allows for very few continuities between the testaments, if any.
Calvinism is historically linked with covenant theology, which stresses St. Paul's account of God's plan in the New Testament: the promise was given to Abraham and his Seed through faith and that promise belongs to all the people of God through Christ. Faithful people of the Old Testament era did not receive the entirety of what God promised because only with us could they reach that completion (since it is through Christ). This point is mentioned in the book of Hebrews. In other words, continuity in God's plan, according to St. Paul, is the significant element, not the distinctions or changes that served as a part of that grand plan. It's a matter of seeing the big picture and understanding that the differences through time become irrelevant, more or less. Dispensationalists prefer to focus on those differences and to hold onto them in spite of St. Paul's conclusion. They foresee a further working out of God's plan even though St. Paul's argument is that all things climax or culminate with the cross. Dispensationalists are usually not Calvinistic because these two systems are not historically linked. Dispensationalists usually stress freewill when it comes to whether or not people believe the gospel. In my opinion it is not necessarilyt unreasonable to mix and match beliefs here and it does happen at times, as with the case of John MacArthur. But it is my belief that Dispensationalism represents a misunderstanding of the overall plan of God.
There are a number of sharp distinctions, but they are definitely not impossible to put together. The most important criteria in being able to mix belief systems is how much we are willing to stretch (as is not stick loyally to) the potentially less truthful parts of one belief system so that they may be covered by the more truthful parts of another belief system. At the very least, we need to be able to see the truthful parts in any belief system we encounter and be willing to adapt our own beliefs to conform to new-found truth.
In the case of Calvinism and Dispensationalism, Calvinists focus on the Covenant, which has continued from Abraham into today, only through different forms. Dispensationalists stress the divisions between time periods and the different ways God deals with his people throughout time. The also strongly disagree on the idea of "God's people"; Calvinists tend to put the Israelites and Christians together as part of the same promise, while dispensationalists will see the two as completely different.
If someone on either sides refuses to see the truth in the other perspective, they can't mesh. One way in which they could work together is to see that God has indeed been using covenants in different forms throughout time, as the Calvinists believe. Then add onto that the realization that the primary interaction between God and humanity has varied through history (there was indeed a time when people primarily relied on their conscience for morality, a time when the patriarchs lived by faith in God's promise, a time when the Israelites obeyed God by following His written law, and now we are in a time where we see that God saves His people through grace).
However, in order for this mixing to even start working, a dispensationalist would need to be able to suspend his belief that those dispensations were the method of salvation for God's people in favor of the idea that all of God's people are saved through faith. In turn, a Calvinist would need to be willing to divide history into stages in the first place. From my experience, people are often unwilling to drop the beliefs they grew up with (or beliefs they came upon during their own research), which is why it's so hard for Calvinists and dispensationalists (which come from very different perspectives) to compromise.
Why is it rare to combine Reformed/Calvinist doctrine and Dispensationalism?
Adam Smith (who was no fan of religion) thought different denominations were great. He saw that in order to coexist in the marketplace of ideas, they would eventually have to drop those things that were distinctive so that they would not produce discord and thus become impotent.
It is the nature of denominations (the word for divisions or disunity in the Greek is hairesis from which we get heresy) to function as an organizational systems or franchise brands. This by itself (definition by distinction) tends to prevent what might be called a harmonization.
There are doctrinal differences that are difficult to surmount. For example the idea that Israel has been replaced or subsumed by the church is not compatible with the idea that a faithful remnant of Israel will be used of God for an eventual kingdom of God on earth.
While at first glance covenants and dispensations might be seen as similar divisions of time, however, what is meant by and what happens in those divisions becomes an insurmountable obstacle.
In addition to the doctrinal distinctions, there are cultural distinctions that not only create barriers but can at times become vitriolic. Covenant (Calvinist) people sometimes come unglued when considering dispensational people because they can see them as lazy and just waiting for God to do something when they think Christians should be actively making things better.
One reason these distinctions are such barriers is that they are often mastered after much study and often those who make such an effort come to be the staunchest defenders of their mastered distinctions.
1 Corinthians 8:1 Now as touching things offered unto idols, we know that we all have knowledge. Knowledge puffeth up, but charity edifieth.
They key to Christian unity is humility (not being "puffed up"). Christian denominations are maintained by those who have gone through an approval process such that they can be counted upon to defend the denominational distinctives. Once a person can make allowance for the fact that he does not understand everything and may in fact understand some things poorly, he is less likely to condemn others for their beliefs.
1 Peter 5:5 Likewise, ye younger, submit yourselves unto the elder. Yea, all of you be subject one to another, and be clothed with humility: for God resisteth the proud, and giveth grace to the humble.
The reason that many if not most Christians cannot get past denominational distinctives can be accounted for by the following;
- Lack of maturity.
1 Corinthians 3:1-3 And I, brethren, could not speak unto you as unto spiritual, but as unto carnal, even as unto babes in Christ. I have fed you with milk, and not with meat: for hitherto ye were not able to bear it, neither yet now are ye able. For ye are yet carnal: for whereas there is among you envying, and strife, and divisions, are ye not carnal, and walk as men?
- Worldly entanglements.
James 4:1-4 From whence come wars and fightings among you? come they not hence, even of your lusts that war in your members? Ye lust, and have not: ye kill, and desire to have, and cannot obtain: ye fight and war, yet ye have not, because ye ask not. Ye ask, and receive not, because ye ask amiss, that ye may consume it upon your lusts. Ye adulterers and adulteresses, know ye not that the friendship of the world is enmity with God? whosoever therefore will be a friend of the world is the enemy of God.
- Diverted faith.
Galatians 3:1-3 O foolish Galatians, who hath bewitched you, that ye should not obey the truth, before whose eyes Jesus Christ hath been evidently set forth, crucified among you? This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith? Are ye so foolish? having begun in the Spirit, are ye now made perfect by the flesh?
- Living in the flesh.
Galatians 5:17 For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
First, keep in mind that neither Dispensationalism nor Reformed/Calvinism fit neatly into tight boxes. There's a great deal of theological/philosophical diversity/variation in both realms. Clarity is important, so I'll strive to keep it as simple as possible.
The best approach to answering the question might be to mention what core beliefs they share in common, before proceeding to their CORE difference(s). (PS. I've spent nearly 55 years exploring these realms. When I reference "Dispensationalism," I historically trace the doctrinal distinctions to the extraordinary John Nelson Darby (JND) and his Classical Pauline view(s). The most definitive resources here are the extensive works of the late Roy A. Huebner.) Say what you will, but Darby was both a stalwart and a warrior for our Lord Jesus Christ.
COMMON: Both Reformed/Calvinism and Classical Pauline Dispensationalism share a belief in the Sovereignty of God (in contrast to the sovereignty of Man) and the epistemological foundation of the Inerrant Word of God--the 66 Books of the Bible. Both believe that Election and Salvation are based on God's Grace and Mercy, apart from ANY form of merit.
DIFFERENCE: Reformed/Calvinism follows traditional Christendom in the use of God's covenants made with earthly Israel to superimpose a forced continuity to the 66 books of Scripture. By contrast, Dispensationalism held this obfuscated the clear evidence of discontinuity and made Christianity an interloper in Israel's unique promises, the prime example being Catholicism.
A marked split with traditional Christendom, Classical Pauline Dispensationalism (JND) held that the covenants (including the briefly-mentioned "New Covenant") exclusively pertained to the earthly Elect Nation of Israel(1). Further, the Church/Body of Christ (whose advent was recorded in Acts 2) was the "heavenly people of God" and a separate group having a level of discontinuity from Israel.
(1) See An Introduction to the NEW COVENANT, Christopher Cone, General Editor.