Is that a Bible quote? I can think of "do not tire out".
In general it's a bad idea to present your message as a negative. If you don't want people to give up it's better to say positively: "keep in the race" or "keep on enduring". If you say "don't give up" all people can think about is "giving up".
If this is the theme, no doubt the full message will be: don't give up because the end is really near and you'll miss out completely if you don't keep doing as we tell you.