Let me simply respond by asking the following questions.
1. If the gentiles were to be chosen by God outside of the preaching and subsequent obedience to the gospel - what need was there for Peter to come and preach to Cornelius and his family?
2. What is the significance of the Jews recognizing that the Holy Spirit was poured out on the gentiles in the same way as in the beginning (Acts 11:15)? Were the Jews at Pentecost saved as soon as the apostles started speaking by the power of the Holy Spirit - or did other events take place that finalized the process?
3. Has anything God has ever commanded been non-essential? If so, which commandments do we choose?
4. Later when recounting the events, why did the Jews note the importance of God granting the gentiles repentance unto life (Acts 11:18)? Did the gentiles repent before or after the Holy Spirit fell upon them?
5. If a person is born of the Spirit only, why did Christ himself included the necessity of both the water and the Spirit (John 3:3-5)?
6. The question was asked, "Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we?" (Acts 10:47). What measure of the Holy Ghost was received? Why was water even brought up in the conversation? If water was not important, why would it matter whether or not it was forbidden?
7. What was the purpose of the Holy Spirit falling upon the gentiles in the same manner it fell upon the apostles on the day of Pentecost?
The Acts 10 account of the gentiles admission into the church displays God's choice to include the gentiles for salvation. It also teaches the same necessity of obedience to the gospel of Christ that was commanded at Pentecost. Just as the Jews were not saved until they responded to the apostles' preaching, neither were the gentiles saved until they responded to the message of Peter. That God had chosen all men for salvation is the exciting reality of this passage.
The Bible does not teach that we are saved by a direct operation of the Holy Spirit. It does teach, however, that the Spirit has revealed the mind of God (1 Cor. 2:9ff), and that when we obey what the Spirit has revealed we can be saved (Acts 2:38-41, 47).
The gentiles in Acts 10 were not saved without baptism. Though they were chosen to receive salvation, they still needed the washing and cleansing that baptism offers through contact with the blood of Christ.
"He who believes and is baptized shall be saved..." ~ Mark 16:16