“But when that which is perfect has come, then that which is in part will be done away.”
The “but” is a conjunction of contrast. The contrast is between the “perfect” and the “in part.” The contrast is between the partial capacities of gifts versus the full-fledged revelation of God.
when that which is perfect has come,
The word “perfect” means complete, mature, full-grown. The word “perfect” is neuter in gender (so cannot refer to Christ) and correlates with “in part.” Whatever the “in part” refers to, the “perfect” is its counterpart or antithesis. Both have the same referent. The contrast is one of quantity. In the church age, the gifts of prophecy and knowledge were partial, Paul was saying, but there would come a time when we would have a full understanding of prophecy and a full-grown understanding of God. Here “perfect” means full truth about God. That day came when the canon of Scripture was complete.
then that which is in part will be done away.
When the full truth about God comes, then there is no need for the partial, delimited gifts of prophecy and knowledge extant today. While the canon was in formation, the church had fragmentary knowledge. Partial gifts are unnecessary with a completed canon.
We have a complete canon, so there is no need for direct revelation.
God replaced partial prophecy and knowledge with full revelation in the canon. This does not mean that Christians will have omniscience but that what we know is accurate knowledge from God.
All that we need is in the Bible. We have it all in writing in the Bible. If you don’t come up with chapter, book, and verse, we don’t believe a word you say.