Were the twelve thrones judging the twelve tribes of Israel a reward or salvation for the disciples?
The twelve thrones were a reward, not salvation. The disciples already had salvation, and the promise concerned what they would receive for serving Christ diligently during His three-year earthly ministry. Matthew 19 presents the thrones as reward for service, not as the means of eternal life.
Les explains that in Matthew chapter 19, the twelve had been with Jesus for almost the whole three years. They already had their salvation—Judas not being counted—and Jesus' promise that they would sit on twelve thrones judging the twelve tribes of Israel was their reward for having served Him so diligently during His earthly ministry. Les stresses that it had nothing to do with their salvation, but everything to do with what they would receive for it all. He then connects this same concept to II Corinthians chapter 5, where Paul is not dealing with salvation but with reward, and verse 10 speaks of appearing before the bema seat.
Then answered Peter and said unto him, Behold, we have forsaken all, and followed thee; what shall we have therefore?
And Jesus said unto them, Verily I say unto you, That ye which have followed me, in the regeneration when the Son of man shall sit in the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.