Adam Clarke on Psalms 52:8

But I am like a green olive-tree in the house of God - I shall be in the house of God, full of spiritual vigor, bringing forth evergreen leaves and annual fruit, as the olive does when planted in a proper soil and good situation. It does not mean that there were olive-trees planted in God's house; but he was in God's house, as the olive was in its proper place and soil.

I trust in the mercy of God - The wicked man trusts in his riches: I trust in my God. He, like a bad tree, bringing forth poisonous fruit, shall be cursed, and pulled up from the roots; I, like a healthy olive in a good soil, shall, under the influence of God's mercy, bring forth fruit to his glory. As the olive is ever green, so shall I flourish in the mercy of God for ever and ever.