The gospel in Mattew didn't have the blood of Christ behind it. That was looking forward. I will build my church. Will build indicates it hasn't been built. The church was purchased with his blood.
Acts 20:28....to shepherd the church of God which He purchased with His own blood.
After his D,B &R, the gospel had his blood behind it. Those who believed were saved.
Mark 16:15 And He said to them, "Go into all the world and preach the gospel to all creation. 16 He who has believed and has been baptized shall be saved; but he who has disbelieved shall be condemned.
As I see it there the truth was not delivered as MAD has it - multiple packages (gospels). It was delivered line upon line, in a way, like a fax. His sacrifice which had been prophesied, was alluded to by John the Baptist who called Jesus the lamb, then by Jesus remarks saying He would die and be resurrected and then by the institution of the Lord's Supper. What the cross was all about could not be revealed before it happened. Otherwise, the Enemy would not have crucified Jesus (
1 Corinthians 2:8)
The truths about Jesus as the unique Son of God, His miraculous power ministry and teaching, that salvation unto eternal life could be obtained through faith in Him were all revealed in the Gospels before the cross. These progressively became a part of the body of truths we now call the "gospel."
After the Resurrection Jesus came to the disciples and explained what He had done and how it fulfilled the Messianic prophecies (
Luke 24:27,44) The same message they had received from Jesus in person Paul received later from Jesus by revelation.
You are certainly right that the Church could not be formed until it was bought by His blood and filled with His Spirit.