A religion is a social organism, so a successful religion will have to balance its own needs with the needs of its followers. From my own experiences with and observation of religion, I think these are the core elements:
Times change! If your religion is too concrete and too obvious about what it means, it runs the risk of falling by the wayside. Christianity has survived as long as it has because of the ambiguity and contradictions it contains. This allows people to build whichever version of your religion best suits the time and place they find themselves in, which increases the odds that one such version will end up surviving for centuries. The only "timeless truth" is that which does in fact stand the test of time. There's no way to tell for sure what that will be, so don't try to pin it down. Just make sure your religion can be interpreted in such a way that it encompasses whatever it may need to encompass.
Growth & Maintenance
Make sure your religion has elements that encourage its numeric growth, as well as elements designed to maintain current members. The Christian Bible has The Great Commission as well as verses like Hebrews 10:25. Scientologists are given the edict to "clear the planet" and famously persecute ex-members. Having regular meetings and events will help build strong social ties between members and make it harder for them to leave.
Most successful religions actually impart good advice to their members that ensure they'll experience greater success than a regular schmuck would. Christianity has stuff like "whatever you do, work at it with all your heart" and "do everything without grumbling or arguing." Some of Scientology's material reads like lifehack articles with batshit crazy stuff mixed in. If your members are not successful in life, your religion won't be either. Note that this doesn't necessarily have to be monetary success: Christianity gained a lot of followers in its early days due to the crazy quantities of courage it imparted to Christians as they were being thrown to the lions. In Rome's socially stratified society, lower-class people demonstrating impressive degrees of internal strength was very notable and persuasive.
Related to the above point, but it deserves its own bullet because getting a high-profile member will raise your religion's profile. Christianity had Constantine, and Scientology has Tom Cruise. And early on, having members who are an asset to your fledgling religion rather than a drain can be very useful. You can either make your members powerful via good advice and social support, or you can obtain members who are already powerful. A mix of both is best.
Your religion has to scratch an itch other things don't scratch. It has to offer catharsis and meaning. The primary value proposition of religion for a lot of people is that it frees them from the work of having to generate their own sense of meaning. Give them rituals, symbols, holidays, practices, or whatever else they like. Give them an outlet for things they can't necessarily let out in other spheres, and give them good rationalizations that let them feel they're simply misunderstood by a misguided world rather than the other way around. They're going to need some pleasure to balance the whip of your religion's good advice. Whatever you do, though, keep it simple. Otherwise your religion will be unlikely to achieve mass popularity.