It is important to consider the verse in context:
1 Timothy 1:15-16 (DLNT):
15 The saying is trustworthy and worthy of full acceptance— that Christ Jesus came into the world to save sinners, ...
Setting aside all personal (theological) theories, the context shows the verse 1 Timothy 1:17 refers to none other than Lord Jesus Christ.
The immediate ‘historico-grammatical’ context, which ...