You have overcome them, because greater is He who is in you than he who is in the world.
Notice the tense first. You have overcome. Past tense, already accomplished, stated to people who were in the middle of something difficult and presumably did not feel victorious.
John is not describing how they felt. He is telling them what is true underneath how they felt, which is the more useful of the two things to know.
Then the comparison, and it is not close. Greater. He does not argue for it or offer evidence. It is stated the way you would state a fact about physics.
The two prepositions matter as well. In you, and in the world. One is internal and permanent. The other is environmental and temporary.
On a day where the opposition feels larger than your resources, that is exactly the sort of sentence worth saying out loud rather than only reading.
The comparison was settled before you woke up.
