For I say, through the grace given to me, to everyone who is among you, not to think of himself more highly than he ought to think, but to think soberly, as God has dealt to each one a measure of faith. Rom 12:3
The Christian has no room for bragging about being a Christian or boasting about special knowledge.
Bragging about Christ is not good for those that do not know Him personally.
Boasting about the superiority of church teaching or spiritual gifts is not good for those in another church.
If it were not for the measure of faith there would be no boasting at all. God does not want us to think highly of ourselves but soberly, that is, be honest of your evaluation of yourself.
Paul refers to the different parts and functions of the body, something he would go into more detail in another letter.
What part of your body is the most important? Your brain? It runs everything but would be useless with nothing to run.
Your feet are very useful for getting you around but not so much when you want to eat a sandwich.
Your hands are handy for the sandwich but not so much when you need to jog on the track.
Just as nothing can brag against the other, neither can one Christian boast against another.
As the Psalmist said, “My soul shall make its boast in the Lord.” Ps 34:2