I understood it immediately as well, but understanding it does not mean it's grammatically correct.
There are several coordinating conjunctions that may be used including and, but, for, nor, or, so, and yet. Which is best to use depends on the relationship between the two independant clauses being joined.
In this case, the second clause is redundant. If you 'are following', then by defintion, you have to be 'not leading'. It is redundant therefore, not contrasting. However, consider the following, which is simply a reversal of the two clauses.
The Pope will not be leading services this Sunday, but he will be following them.
The difference is that 'not leading' does not mean that you are following. You can be doing nothing. Therefore, you have a clause of what the Pope is not doing, contrasted with a clause about what he is doing.
Good Luck
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