1 Now the apostles and brethren who were in Judea heard that the Gentiles had also received the word of God.
2 And when Peter came up to Jerusalem, those of the circumcision contended with him,
3 saying, “You went in to uncircumcised men and ate with them!”
4 But Peter explained it to them in order from the beginning, saying:
5 “I was in the city of Joppa praying; and in a trance I saw a vision, an object descending like a great sheet, let down from heaven by four corners; and it came to me.
6 When I observed it intently and considered, I saw four-footed animals of the earth, wild beasts, creeping things, and birds of the air.
7 And I heard a voice saying to me, ‘Rise, Peter; kill and eat.’
8 But I said, ‘Not so, Lord! For nothing common or unclean has at any time entered my mouth.’
9 But the voice answered me again from heaven, ‘What God has cleansed you must not call common.’
10 Now this was done three times, and all were drawn up again into heaven.
11 At that very moment, three men stood before the house where I was, having been sent to me from Caesarea.
12 Then the Spirit told me to go with them, doubting nothing. Moreover these six brethren accompanied me, and we entered the man’s house.
13 And he told us how he had seen an angel standing in his house, who said to him, ‘Send men to Joppa, and call for Simon whose surname is Peter,
14 who will tell you words by which you and all your household will be saved.’
15 And as I began to speak, the Holy Spirit fell upon them, as upon us at the beginning.
16 Then I remembered the word of the Lord, how He said, ‘John indeed baptized with water, but you shall be baptized with the Holy Spirit.’
17 If therefore God gave them the same gift as He gave us when we believed on the Lord Jesus Christ, who was I that I could withstand God?”
18 When they heard these things they became silent; and they glorified God, saying, “Then God has also granted to the Gentiles repentance to life.”
Apparently the apostles in Jerusalem had only heard part of the story of what happened at Cornelius’ house
those of the circumcision (v.2) — some commentaries say this refers to Jews who considered circumcision and obedience to the law to be of prime importance — like the Judaizers in Galatians — but I don’t think the context makes a distinction between them and those mentioned in verse 1.
contended (v.2) = repeated and prolonged doubting and division
ate with them (v.3) — they seemed to consider this Peter’s chief offense — as in Galatians 2:11-14
doubting nothing (v.12) — Peter’s marching orders
at the beginning (v.15) — at Pentecost
Peter’s mission to Cornelius was a unique case designed by God for a purpose then still unrevealed.
A passage from J.N. Darby’s Bible Synopsis may be helpful here: “Now the mystery had been hidden in all former times; and in fact it needed so to be; for to have put the Gentiles on the same footing as the Jews would have been to demolish Judaism, such as God had Himself established it. In it He had carefully raised a middle wall of partition. The duty of the Jew was to respect this separation; he sinned if he did not strictly observe it. The mystery set it aside. The Old Testament prophets, and Moses himself, had indeed shown that the Gentiles should one day rejoice with the people; but the people remained a separate people. That they should be co-heirs, and of the same body, all distinction being lost, had indeed been entirely hid in God … ” (Acts to Philippians, pgs. 431-432)
The objection of the circumcision saints at Jerusalem, then, was simply that Peter had departed from the revealed order and program of God, for he, like they, had been sent to carry out a program consistent with the Abrahamic covenant and with prophecy. — Stam, page 104.