In SEL, God is defined most loosely as an omnipresent being who has at least one follower, much like Eiri when he is claiming to be God of the Wired, and most narrowly as an omnipresent, omnipotent and omniscient being, much like Lain at the end of the anime.
[...God.] --What god? [The God of Wired.] --There's a God of Wired? You believe that? [(silence)] --Answer me. [...There is if I believe there is...] --That answer isn't good enough. [...There is...a God...] --All right, fine. The God of Wired is the god of the top level of the real world hierarchy--in other words, a being that controls the entire world. Is that right? [...Who are you?]
The interviewer is suggested to be Eiri in disguise, speaking to Lain by a means similar to that seen in Layer 05, in which he uses a holographic image of a doll, Lain's mother Miho and Lain's father Yasuo to speak to and influence Lain.
Lain's statement "There is if I believe there is" can be seen not as a means of avoiding the question but as a reflection of the state of affairs later, in Layer 10. All the Knights of the Eastern Calculus, Eiri's followers, commit suicide and Lain claims that Eiri is no longer God because he has no remaining followers. However, Eiri responds that there is one believer left, so he can remain God. That believer is Lain. This way, whether or not Eiri is God of the Wired truly depends on whether Lain believes in him or not.