Now if they believe in eternal existence. Then what is the difference between them & creationist & believing in an eternal God.
Specificity. Your previous paragraph gave a perfectly logical argument against there being some definitive origin which makes believing that there has always been
something in existence perfectly rational. Once you start making any kind of specific and detailed assertions about what that "something" is without any evidential or logical basis though, you're going beyond the rational and in to faith.
A couple of side-points from your OP;
There's not really any such thing as an "evolutionist". It's just one of countless sub-fields of science and nothing that special or worthy of specific focus. The general beliefs and opinions of all the people who causally accept evolution as a reasonable explanation for the diversity of life and that of the much smaller group of people who have actually studied the field will be largely independent of that specific aspect. There are even people who believe in some kind of Creator but also in evolutionary process.
You also seem to presume that all Creationists are Christians (and a specific type of Christian at that). This is obviously not the case and so suggests a major flaw in your perceptions which you might wish to reconsider.