We have forsaken all - Probably nothing but their fishing-nets, small boats, and cottages.
But they were their all - their living, their home; and, forsaking them, they had as really shown their sincerity as though they had possessed the gold of Ophir and lived in the palaces of kings.
What shall we have, therefore? - We have done as thou didst command this young man to do. What reward may we expect for it?

We have forsaken all - "A poor all," says one, "a parcel of rotten nets." No matter - they were their All, whether rotten or sound; besides, they were the all they got their bread by; and such an all as was quite sufficient for that purpose: and let it be observed, that that man forsakes much who reserves nothing to himself, and renounces all expectations from this world, taking God alone for his portion. See Matthew 4:20.
To forsake all, without following Christ, is the virtue of a philosopher. To follow Christ in profession, without forsaking all, is the state of the generality of Christians. But to follow Christ and forsake all, is the perfection of a Christian.
What shall we have therefore? - Τι αρα εϚαι ημιν, What Reward shall we get? This Kypke proves to be the meaning of the words from some of the best Greek writers.

Then answered Peter and said unto him,.... Peter observing what Christ said to the young man, bidding him sell all that he had, and give to the poor, and he should have treasure in heaven, and come and follow him, lays hold on it, and addresses him in the following manner,
behold, we have forsaken all, and followed thee. Though their worldly substance was not so large as the young man's, they had not such estates to sell, nor that to give to the poor, he had; yet all that they had they left for Christ's sake, their parents, wives, children, houses, and worldly employments, by which they supported themselves and families; and became the disciples and followers of Christ, embraced his doctrines, submitted to his commands, imitated him in the exercise of grace, and discharge of duty, denying themselves, and suffering many hardships on his account: wherefore it is asked,
what shall we have therefore? what reward for all this? what part in the Messiah's kingdom? or what treasure in heaven?

Lo, we have left all. Compare Mark 10:28-29, and Luke 18:28-30. The apostles had left all they had for Christ. The case of the rich young man suggested Peter's question.