The "good Thief" had the same access to the OT scriptures that the apostles had..
The OT saints were saved by believing the promise of the scripture and looking forward to the cross..
Remember that the Old Testament was in effect until Christ died on the cross... so the thief was saved on the promise just like Moses , David and the other OT saints
Additionally, the publican was saved by crying out, “ God be merciful to me, a sinner.”
And there were some people to whom Jesus said, “Your faith has saved you. Go in peace.”. and I don’t recall that any of them said or did some church or religious thing.
Because it’s the response of the heart to the call of God.
He meets us where we are, as we are.
We don’t have to clean ourselves up, go somewher special, do something special. Just cry out to Him. He sees our heart and responds to that.
They have Moses and the prophets.
Let them listen to them.