The key

Knowledge and discipline °are °°power.

° This link will explain why you are required to use are, and not is:

https://owl.english.purdue.edu/owl/resource/599/01/

°° Power can be singular in this sentence, but it can also be plural if you want to change the meaning.

In your example you are saying that knowledge and disipline, together, are what define power. That is, power is a result of those two things combined. However, if you use the plural, powers, then you are saying that they are both individual powers, and not necessarily related. 

Examples:
: What is power? Power is knowledge and discipline.

: What powers do you have? I have the powers of knowledge and displine. 

The Key

Knowledge and discipline is a power.

The key

knowledge and discipline is power