MacArthur wasn't guessing when he said what he said. It's grounded in the bible. There is biblical precedent for his statement.
God instituted a Doctrine of Works. Don't bristle. Stay with me. In Genesis 2, God told Adam,
The LORD God took the man and put him in the garden of Eden to work it and keep it. 16And the LORD God commanded the man, saying, “You may surely eat of every tree of the garden, 17but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die.” (Genesis 2:15-16).
As Martyn Lloyd Jones said of the Doctrine of Works in his sermon "The Covenant of Grace in the Old Testament", the Genesis verses are obviously a doctrine of works. God told Adam to--
--work the Garden
--keep the Garden
--not eat of the fruit.
The inheritance of the reward from God was entirely dependent upon what Adam did or did not do. This is works.
It failed.
It failed immediately and utterly. Adam failed to work correctly for his reward from God. Since we are all in Adam from the moment of birth, from the moment of conception even, (Romans 5:12, Psalm 51:5) we, too, will fail to please God with our works. (Hebrews 11:6).
Once again Lloyd Jones, "If man in a perfect position didn't keep the covenant of works, then what is the point of God making a new Covenant of Works? And indeed He didn't. He then made a Covenant of Grace."We cannot, cannot, inherit any reward from God based on our own works. We have proved this. It was tried, it failed, it's done. God made a Covenant of Grace which is that we receive a reward from Him based on HIS choice, HIS will, HIS election, HIS grace. Our reward is all based on faith, and guess what? The faith we have is also a granted gift from Him. (Galatians 3:22). It is a faith that HE keeps for us and in us. It's sealed. (2 Corinthians 1:22, Ephesians 1:13).
This knowledge of works v. faith has import for those who are in a works-based religion - which is to say everyone else in all other religions, even atheism. Mormons try to reach god through their covenant of works. Catholics try to pile up works so as to acquire enough to please God. Islam teaches "To those who believe and do deeds of righteousness hath Allah promised forgiveness and a great reward," (Surah 5:9). And so on. And so on. And so on...
Our working out of our salvation through fear and trembling is a result of the sovereign choice of God to dispense faith and repentance to us. It's based on our knowledge of the above, that our works while in the flesh count as filthy rags. It's all Jesus, from start to finish, including His reward to us. We receive a glorified body so that we may no longer sin against Him. We become sons, adopted to His family, and thus co-heirs. We receive manifold and eternal mercies in heaven. Most of all we receive HIM. Jesus is our treasure. In His grace he shared Himself with us, gladly, voluntarily.
Praise God for the Covenant of Grace. Because, if I could lose my salvation, I would.