[be] (only|just|) a matter of time

to certainly happen at some point in the future

The idiom "be a matter of time" is used to convey the idea that a particular event or outcome is inevitable and will occur in the future, although the specific timing may be uncertain. Its origin is not precisely documented, but it has been used for many years to indicate the inevitability of a situation.

With the team's talent and dedication, it's a matter of time before they win the championship.
Given the rapid advances in technology, it's only a matter of time before autonomous vehicles become a common sight on the roads.
For a budding entrepreneur with a great product, it's just a matter of time before their business takes off.
With the global trend towards renewable energy, it's only a matter of time before fossil fuels become less dominant.
The team's hard work and innovation have positioned them for success; it's a matter of time before they secure a major contract.