The problem with infinity is that most people cannot comprehend the concept. For example, there is the infinite set of integers and the infinite set of real numbers between each and every integer.
So the infinite set contains an infinite set of infinite sets. A lot of people would consider this fairly useless mental masturbation, however, understanding the difference between infinity and practical limits is wicked critical.
For example, mathematically if we start at zero and calculate the distance between point A and point B as "D" and attempt to use division, as in B=(B-A)/2, to define the steps necessary to reach B we enter the infinite set between the finite set of (A,B) because we are using that infinite set as a function, or a portion, of the finite set.
In reality though what we have done is confused units from the finite to the infinite.
It is like the old joke about the engineer and the mathematician, the math guy can't talk to a pretty girl because he moves towards her in a proportional function of the finite distance which results in a curve that can never achieve intersection.
The engineer uses an interval function which results in a straight line that intersects.
The engineer meets the girl while the mathematician becomes stuck in an infinite loop.
So the question is, when accelerating do we move based on the addition of energy or do we move based on the multiplication of a proportional velocity or mass?
In fact, since energy is mass, as we expend energy to achieve velocity we reduce mass because we are working, from a practical application, with the addition of energy and the subtraction of mass rather than the multiplication of proportional velocities and proportional masses which can only result in an infinite mathematical loop.
And that is why Einstein is full of shit.