Allah says,
{And verily, those before them did devise plots,}

against their Messengers, they wanted to expel them from their land, but Allah devised plots against the disbelievers and gave the good end to those who fear Him. Allah said in other Ayat,
{And (remember) when the disbelievers plotted against you to imprison you, or to kill you, or to get you out; they were plotting and Allah too was plotting; and Allah is the Best of those who plot.} [8:30], and,
{So they plotted a plot, and We planned a plan, while they perceived not. Then see how was the end of their plot! Verily, We destroyed them and their nation, all together.} [27:50,51]
Allah said next,
{He knows what every person earns,}

meaning, He alone knows all secrets and concealed thoughts and will reckon each person according to his work,
{and the Kafir (disbeliever) will know}

or the Kuffar (disbelievers) according to another way of reciting,
{who gets the good end.} who will earn the ultimate and final victory, they or the followers of the Messengers. Indeed, the followers of the Messengers will earn the good end in this life and the Hereafter, all thanks and praise is due to Allah.
{And those who disbelieved, say: "You are not a Messenger."
Say: "Sufficient as a witness between me and you is Allah and those too who have knowledge of the Scripture."}

meaning, the disbelievers reject you and say that you are not a Messenger from Allah. Say to them, 'Allah is sufficient as a Witness between me and you. He is Witness over me, knowing what I brought you from Him, and Witness over you and the lies and falsehood you invent.'
{and those too who have knowledge of the Scripture.}

this refers to 'Abdullah bin Salam, according to Mujahid. However, this is a Makkan Ayah and 'Abdullah bin Salam embraced Islam after the Prophet emigrated to Al-Madinah. The correct opinion is that this Ayah refers to the scholars among the People of the Scriptures who find the description of Muhammad in their Books and the good news of his coming that were delivered to them by their Prophets. Allah said in other Ayat,
{And My mercy embraces all things. That (mercy) I shall ordain for those who have Taqwa, and give Zakah; and those who believe in Our Ayat; Those who follow the Messenger, the Prophet who can neither read nor write whom they find written with them in the Tawrah and the Injil} [7:156-157] and,
{It is not a sign to them that the learned scholars of the Children of Israel knew it (as true)?} [26:197]
There are similar Ayat that affirm that the scholars of the Children of Israel know this fact from their divinely revealed Books.
This is the end of Surat Ar-Ra'd, and all praise is due to Allah and all favors are from Him.