I assume that the first sentence is there for a reason, and that you wish to understand why it allows both forms while the second allows only one. The reason is that insist has two meanings, or two shades of meaning, if you will. If you insist that someone goes to Athens, you are saying quite firmly that you know that the person makes regular trips to and from Athens.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
This meaning of insist expresses serious possibility, does it not?No. Not to me. It expresses conviction and certainty.