Let’s read Exodus 12:37-38 so that we might answer your interesting question.  That says, “And the children of Israel journeyed from Rameses to Succoth, about six hundred thousand on foot that were men, beside children. And a MIXED MULTITUDE went up also with them; and flocks, and herds, even very much cattle.” 

This describes the event of the Israelites leaving Egypt after their years of slavery in Egypt.  We read in verses 40-41, “Now the sojourning of the children of Israel, who dwelt in Egypt, was four hundred and thirty years. And it came to pass at the end of the four hundred and thirty years, even the selfsame day it came to pass, that all the hosts of the LORD went out from the land of Egypt.” 

In verse 38, we learn that some people left with the Israelites.  Who were these people?  Why did they join the Israelites in their exodus from Egypt?  Historically, the Israelites typically had servants.  However, there are no servants mentioned in their migration to Egypt.  As servants to the Egyptians, we might assume, at this point of their own slavery, they had no slaves of their own to take with them as they left the land.  It seems plausible that these people were discontented Egyptians, or perhaps some who had settled in Egypt because of their fertile lands. 

We read later of the bad influence of this mixed multitude in Numbers 11:4-6, “And the mixt multitude that was among them fell a lusting: and the children of Israel also wept again, and said, Who shall give us flesh to eat? We remember the fish, which we did eat in Egypt freely; the cucumbers, and the melons, and the leeks, and the onions, and the garlick: But now our soul is dried away: there is nothing at all, beside this manna, before our eyes.”  These people complained about the food the Lord had provided and longed for the food of Egypt.  It seems to show that their hearts and longings were still in the land they left. 

The only other time we read of a “mixed multitude” was during the re-building of the walls of Jerusalem after their captivity in Babylon.  We read in Nehemiah 13:1-3, “On that day they read in the book of Moses in the audience of the people; and therein was found written, that the Ammonite and the Moabite should not come into the congregation of God for ever; Because they met not the children of Israel with bread and with water, but hired Balaam against them, that he should curse them: howbeit our God turned the curse into a blessing. Now it came to pass, when they had heard the law, that they separated from Israel all the MIXED MULTITUDE.”  This shows us that the Ammonites and the Moabites had affiliated themselves with the Israelites and needed to be separated from because they were the enemies of the Lord and His people. Seeing that these instances of mixed multitudes produced negative actions, I think we can safely assume these people were not believers on the Lord, but only traveled with the Lord’s people in order to have a better life for themselves. 

What can we learn from this that we can apply to our own lives as Christians?  Let’s read Jude 4, “For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ.”  There are those who profess to believe in the Lord, but they do not actually have faith in Him.  These people may have “crept in unawares” to lead the Lord’s people away from following Him, they may have crept it to “fleece the flock” by gaining large sums of money from them.  We need to be so cautious about those we fellowship with.  We are cautioned in 2 Corinthians 6:14, “Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?”  (CC)  (564.2)