Concerning the necessity of an outward response to an inward faith and repentance, are you saying that any response is appropriate, or specifically that one must be water baptized to be saved? Also, do you believe that one must be water baptized according to some specific formula -- full immersion, in the name of Jesus or in the name of the Father, the Son, and the Holy Spirit, and specifically to be saved or is just obedience ok?
Also, how do you explain the example of Cornelius and his household? When Peter recounts the story to the apostles and elders in Jerusalem, he notes that an angel appeared to Cornelius and said for them to send for Peter who would bring them a message through which Cornelius and his household would be "saved" (Acts 11.14). And then as Peter spoke, Cornelius and his household received the gift of the Holy Spirit.
Cornelius and his household receiving the gift of the Holy Spirit was then understood as them being accepted by God and God granting them eternal life because of their faith and repentance (11.15-18). And of course, when Peter was preaching to them he said that forgiveness of sins was connected with faith in Christ (10.43). When they heard this message, they apparently believed, received the forgiveness of sins, were accepted by God, saved, and received the gift of the Holy Spirit.
Of course, they were water baptized based upon their salvation, but it was after they were saved and had received the gift of the Holy Spirit. So apparently salvation is dependent upon faith and repentance and not some specific outward response, even water baptism.
Of course, some discount this scripture saying that it was a "special" situation to reveal that the Gentiles were acceptable to God and even added to the community of faith, the church, without having to become Jewish. God certainly did use this event to do that, but I believe that it also reveals that salvation, being born of the Spirit, comes through faith and repentance before any outward expression of faith; whether it be repeating the sinner's prayer, confession of one's faith, water baptism, or even speaking in tongues like Cornelius did. So, salvation is by grace through faith and faith is not even from ourselves, but is a gift from God!
Unity then is based on recognizing as brothers and sisters in Christ those who have faith in Jesus Christ.