Acts 10 describes God-given illumination granted by visions.
A Roman army officer who was a Gentile convert to Judaism. As yet not knowing Christ, but a God-fearer. He showed moral earnestness - simple faith - genuine humility, and was charitable to those in need.
He was given more light. An angel came in a vision, to instruct him regarding a man through whom he could obtain full gospel light. Interestingly, the angel did not bring further gospel illumination, but pointed to a preaching man who could.
Peter is taught that all men, Jew and Gentile alike can be saved. When God's plan is not being acted upon, or even taught, the Lord can, by vision, illuminate that part of the neglected plan, or truth, and cause His word to be acted on.
He became open to the clear specific voice of the Holy Spirit. Visions are intended to do this. The Spirit said to Peter,
The vision prepared Peter to obey the Spirit. He would see the Lord's timing is always perfect. Peter obediently travelled the 38 miles, knowing that from the incredible guidance of Cornelius and of himself by vision and voice, this was the Lord's leading - he took six
brethren with him - proving his obedience to the Holy spirit's illumination. This action in taking witnesses was an indication of wisdom. He was glad of those witnesses later when the whole leading was reported by him at Jerusalem.
Paul rehearses his persecution of the saints, and part in Stephen's death. Visions are given to confirm calling and ministry just as Paul's Gentile calling was re-affirmed.
4.3.1. AN ANGEL APPEARS IN A VISION TO PRAYING CORNELIUS (ACTS 10:3)
What Description Do We Have of Cornelius, as to His Spiritual Condition?
Acts 10:2 "A devout man, and one that feared God with all his house, which gave much alms to the people, and prayed to God alway."
(KJV)
What Happened to Cornelius, Who Lived According to The Light He Had Received?
4.3.2. A VISION IS GIVEN TO PETER PRAYING ON THE HOUSETOP OF HIS LODGING (ACTS 10:10-16)
What Was The Subject of This Instructing Vision?
What Was The Immediate Effect of The Vision on Peter?
Acts 10:19,20 "While Peter thought on the vision, the Spirit said unto him, Behold, three men seek thee. 20 Arise therefore, and get thee down, and go
with them, doubting nothing: for I have sent them." (KJV)
Acts 11:1,2 "And the apostles and brethren that were in Judaea heard that the Gentiles had also received the word of God. 2 And when Peter was come
up to Jerusalem, they that were of the circumcision contended with him," (KJV)
Acts 11:12 "And the Spirit bade me go with them, nothing doubting. Moreover these six brethren accompanied me, and we entered into the man's
house:" (KJV)
Acts 11:18 "When they heard these things they became silent; and they glorified God, saying, "Then God has also granted to the Gentiles repentance
to life." (NKJV)
What Were The Results of Peter Acting on The Vision And Voice of The Spirit?

There is always reward for obedience to the vision and the voice of the Spirit. Peter's was great. 
He was warmly welcomed by Cornelius and his household. 
His ministry was supernaturally confirmed by a mighty outpouring of the Spirit on the Gentiles. 
He started the fulfilment of the prophecy of the risen Lord that they be His witnesses to the end of the earth.
(Acts 1:8 ). 4.4. VISIONS AND CONFIRMATION
HOW WAS PAUL'S CALLING CONFIRMED AFTER HIS LEAVING DAMASCUS?
Acts 22:17-21 "And it came to pass, that, when I was come again to Jerusalem, even while I prayed in the temple, I was in a trance; 18 And saw him
saying unto me, Make haste, and get thee quickly out of Jerusalem: for they will not receive thy testimony concerning me. 19 And I said, Lord, they know that I imprisoned and beat in every synagogue them that believed on
thee: 20 And when the blood of thy martyr Stephen was shed, I also was standing by, and consenting unto his death, and kept the raiment of them that slew him. 21 And he said unto me, Depart: for I will send thee far hence
unto the Gentiles." (KJV)