The only salvation God gives is eternal - that means it lasts forever. It is also given freely, without works or merit - we could never earn it - that means we can also never lose it.

It is not presumption to believe what God says - and the Bible teaches that if someone rejects the truth and turns from it (apostasizes), they were never saved to begin with.

1 John 2:19 They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.

Even this parable shows that those who later turned away never truly believed in their heart - it was a shallow faith, headknowledge.

Luke 8:6 And some fell upon a rock; and as soon as it was sprung up, it withered away, because it lacked moisture.

Luke 8:13 They on the rock are they, which, when they hear, receive the word with joy; and these have no root, which for a while believe, and in time of temptation fall away.

Matthew 13:20-21 But he that received the seed into stony places, the same is he that heareth the word, and anon with joy receiveth it; Yet hath he not root in himself, but dureth for a while: for when tribulation or persecution ariseth because of the word, by and by he is offended.