Why should saving faith also give you the assurance that you are God's and will be in heaven?
Saving faith rests on God's finished work and promises. The same faith that brings a person into salvation should accept God's word that they are His. Since salvation is entirely what God has done, no one can lay a charge against God's elect or take them out of His hand, giving assurance of heaven.
Les teaches that once God has elected us, we have responded, and we have entered into His salvation, nobody can touch that because it is all what God has done. He stresses that if your faith is sufficient to believe the Gospel and bring you into salvation, then you should have enough faith to take God at His word about the rest: you are His, and no one can take us out of His hand. The person who insists no one can know they will go to heaven is actually unaware of true saving faith. Verse 33 asks who can lay anything to the charge of God's elect, and the answer is no one. Les also reasons that God would not elect someone who would later reject Him.