From Thomas Watson, the first puritan I ever read.
What is assurance? It is not any vocal or audible voice, or brought to us by the help of an angel or revelation. Assurance consists of a practical syllogism, in which the Word of God makes up the major proposition, conscience the minor, and the Spirit of God the conclusion. The Word says, ‘He that fears and loves God is loved by God;’ there is the major proposition; then conscience makes the minor, ‘But I fear and love God;’ then the Spirit makes the conclusion, ‘Therefore thou art loved of God;’ and this is what the apostle calls ‘the witnessing of the Spirit with our spirits, that we are his children,’ Romans 8 v 16. A Body of Divinity, BOT, p251
So it’s not that assurance of salvation is impossible to attain, or that a person can never know for sure if they’re saved, but the conclusion can’t be drawn at their own will, or on the basis of what your conscience says without corroboration.