One thing to note is that MLJ was an opponent of the mid-20th century efforts of ecumenicalism. That is, he believed any true Gospel-preaching pastor and church should not willingly stay connected to pastors and churches who had denied the Gospel. At the time, this was very unpopular. Godly men, like John Stott and J.I. Packer, believed evangelicals needed to stay connected to bodies like the Church of England to be salt and light. MLJ, however, spoke vehemently against those who tried to align with churches and pastors who denied the accuracy and authority of the Bible, the substitutionary atonement of Christ, and the veracity of miracles (like the virgin birth, Christ's resurrection, etc.).
The major reason for this view was that MLJ believed you couldn't be in fellowship like this with people who were clearly not Christians. They might call themselves "Christians," but to remove themselves from historic Christianity meant they were Christians in name only. What fellowship could then be had between light and darkness?
I give all this background to make a few simple points:
1) Packer and Stott believed the presence of evangelicals in denominations filled with heretical pastors and teachers would bring about an evangelical renaissance. History has not proven this to be true. In fact, several "evangelicals" abandoned Gospel-truths in order to advance up the church ladder.
2) The spirit of the age in the 20th century was all about unity, ecumenicalism, and the like. MLJ had the courage to go with Scripture rather than public opinion. It is wise to be on "the side of history." But history is not decided by textbooks or opinion-makers but by the holy Judge--the Triune God.
3) As we move forward as Christians in the 21st century, our goal is not to align with the spirit of the age or be on what humans believe is "the side of history." We live before a holy Judge and live in light of His holy statutes. Beware of making decisions that seem wise or winsome to the world if these decisions are not in alignment with Biblical truth. So says Isaiah, "The grass withers and the flowers fade, but the Word of our God lasts forever."