Reformed Christian. I was raised in a Christian family, and always believed in the basic concepts of God, heaven, hell, etc. But I mistakenly thought Christianity was about trying to be "good enough" for God until my mid teens. Around this time I realised that I couldn't be perfect, which was super distressing for a time. But then I read Ephesians 2:8-9 which says:
For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— not by works, so that no one can boast.
This was a big relief, as it meant that I didn't need to rely on trying to be good enough for God. I just needed to accept God's free gift of salvation. That's the moment I would say I became a Christian.
Since then, I've had times where I've questioned it all, but I always come back to the resurrection of Jesus. I find the non-miraculous explanations of the resurrection account to be so implausible that it makes more sense to accept that it's a historical fact. And if the resurrection's true, then it makes sense to believe the rest of it as well.
It's not so much that I believe it 'by default'. Rather, when I've examined the historical case for the resurrection, the arguments that it really happened seem stronger than the arguments that it was a hoax, or a mass hallucination, or that he fainted etc.