The phrase "must also be" has the sense of "it being necessary." Paul understands factions as God-ordained because he could see the pattern of them from Old Testament times, as well as the benefits derived from them.
The Corinthian congregation was a troubled group divided into factions by heresies (I Corinthians 1:10-13). This circumstance was not helping the already-calamitous situation, but Paul says that the calamity would eventually produce a good result. The true sons of God would be revealed by their reactions to the false teaching. They would not accept it, and thus would be witnesses to the weaker for the truth of God.
John W. Ritenbaugh
God's Sovereignty and the Church's Condition (Part Two)
The main point of this entire passage is that those who partake of these Passover symbols should be "discerning the Lord's body" (verse 29). The apostle tells us how to discern the Lord's body in these same verses.
Discerning Christ's Broken Body
Other Forerunner Commentary entries containing 1 Corinthians 11:19:
1 Corinthians 11:23-29