How did the early church interpret St. Paul in Romans 9? Is there a consensus in early Christianity regarding the relation between predestination and free will?
The most important key in resolving an apparent conflicting passage in Scripture is by comparing it with how the same Holy Spirit guiding the early Christians throughout time interpreting the passage being disputed. If throughout time from the earliest moment a dispute was brought in attention and discussed down to the modern time show converging thoughts or cohesive consensus then that view has received the longest attention compared to others.
A popular ecclesiastical view that compatible with St. Augustine of Hippo and St. Thomas of Aquinas doctrine on predestination and free will comes from a saint well respected by Catholics and Eastern Orthodox alike, a monk by the name John from Damascus in Syria. Instead of prioritizing free will by undermining predestination which leads to Pelagian heresy or emphasizing predestination at the expense of free will, this monk explains how the two are synergistically relatable.
One should know that it is customary for sacred Scripture to call God’s permission His action, as when the Apostle says in his Epistle to the Romans: ‘Or hath not the potter power over the clay, of the same lump, to make one vessel unto honor and another unto dishonor?’ He does indeed make both the one and the other, because He is the sole Creator of all things, but it is the own deliberate choice of each and not He that makes them honorable or dishonorable. This is also clear from what the Apostle himself says in his Second Epistle to Timothy: ‘In a great house there are not only vessels of gold and silver, but also of wood and of earth: and some indeed unto honor, but some unto dishonor. If any man therefore shall cleanse himself from these, he shall be a vessel unto honor, sanctified and profitable to the Lord, prepared unto every work.’ It is clear that this cleansing is done freely, for he says ‘if any man shall cleanse himself,’ the converse of which rejoins that, if he does not cleanse himself, he will be a vessel unto dishonor, of no use to Lord, and only fit to be broken. Thus, the foregoing quotation and that which reads: ‘God hath concluded all in unbelief’ and ‘God hath given them the spirit of insensibility; eyes that they should not see and ears that they should not hear,’ are none of them to be taken in the sense of God acting, but in that of God permitting because of free will and because virtue is not forced.
St. John of Damascene, An Exact Exposition of the Orthodox Faith, Book 4, Chapter 19.
The focus that Damascene pointed out is in the aspect of providence namely divine permissive will. It is by focusing on God's providence St. Damascene was able to reconcile the two apparent contradiction harmoniously.