What did the thief on the cross believe? Did he trust in the Gospel?
Yes, he did. The thief knew the Bible. By some point during his crucifixion he realized that this Man next to him was the promised Messiah that the Old Testament Scriptures promised.
How do I know this?
He called Jesus "Lord" - which is the OT name for God, and said remember me when you come into your kingdom (ie. Messianic Kingdom - and the OT only refers to a literal, physical kindgom with the King reigning from Jerusalem) - which He would have to be resurrected in order to do.
He acknowledged the sinlessness of Christ - this man hath done nothing amiss (wrong).
He knew that only God could forgive sins, yet he turned to the Messiah to save him (that requires a belief that the Messiah would be God, as the OT clearly states).
So there we have a belief in the Gospel (ie. the death, burial and resurrection of the Saviour), the Trinity (obviously if Jesus was God, there has to be a Trinity), His complete atonement for sin and His shed blood, His virgin birth and sinlessness, the truth of the Scriptures (as this is where he would have gotten these beliefs from), and also the fact that Christ was coming back to reign physically in an earthly kingdom.
My first few years after coming to Christ for salvation were spent in the Alliance Church. They are ecumenical and allow a lot of heresy and watering down of the Bible.
In discussion regarding the Gospel (ie. exactly what the contents of it was), many times I was told the thief didn't believe such and such (or it wasn't specifically addressed in his conversation with Jesus), or the women at the well didn't believe such and such - so therefore it is not essential to believe in ... in order to be saved. Thus the Gospel was reduced to the absolute lowest common denominator! This fit in with their ecumenical stance.
In 1998 I came to a solid IFB church that emphasized the preaching of the Gospel in every service and during soulwinning times. It was so clear - the Gospel is the death, burial and resurrection of Jesus Christ according to the Scriptures!
1 Corinthians 15:1-4 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain. For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; And that he was buried, and that he rose again the third day according to the scriptures:
I remember in 1999 studying through the book of Acts to see what Peter, Paul, and the other apostles emphasized whenever they preached or witnessed to someone. I admit, after being somewhat brainwashed, I was surprised to find that every single time they preached (that we have the sermon recorded in the Bible) they preached the Gospel, who Jesus was (the Messiah, God manifest in the flesh), and the fact that He was the Saviour who died for their sins, shed His sinless blood, was buried and physically rose again. Every single time! Depending on who they were talking to (Jews or gentiles) they sometimes had to go into greater explanation, but they always dealt with these fundamental truths!
Then I started examining the thief on the cross, the Samaritan women, Nicodemus, and I realized that these things were also mentioned (directly or indirectly) in the passages in some way or another.
The Gospel hasn't changed. It was preached to Abraham (see Galatians 3:8) and others in the Old Testament, it was preached by the New Testament church, and it should be preached by us today.
(This is something I had written years ago - don't remember exactly when - and had saved as a draft. I decided to edit it slightly today and post it.)