As far as I remember this was one of the two strong professions of faith, the second was the Lords Supper, correct me if I am Wrong.
Matthew 18:19 Again I say unto you, That if two of you shall agree on earth as touching any thing that they shall ask, it shall be done for them of my Father which is in heaven. 20For where two or three are gathered together in my name, there am I in the midst of them.
What Jesus is saying there is only that when there is a gathering of believers, even that few, He would be present.
But that's not what makes one a Christian.
The term *Christian* was first used in Antioch to describe believers.
Acts 11:26 ...and when he found him, he brought him to Antioch. So for a whole year Barnabas and Saul met with the church and taught great numbers of people. The disciples were called Christians first at Antioch.
The profession of faith that essentially makes one a believer is that of acknowledging Jesus as Lord.
Romans 10: 9 That if you confess with your mouth, "Jesus is Lord," and believe in your heart that God raised him from the dead, you will be saved. 10For it is with your heart that you believe and are justified, and it is with your mouth that you confess and are saved.
I'm not sure what you are referring to as the strong profession of faith at the Last Supper. There are whole chapters of material about it and what Jesus said there.