Let love be without dissimulation. Abhor that which is evil; hold to that which is good.
We all know how the ivy clings to the wall or to the tree, casts out innumerable little arms and tentacles by which it attaches and fastens itself to it, seeking to become one with it, to grow to it, so that only by main force the two can be torn asunder. It is something of this kind which is meant here. In such fashion cleave to that which is good; and if "to that which is good," then, as the sole condition of this, to Him that is good, who is the Good, the Holy, the Just One.
Parallel VersesKJV: Let love be without dissimulation. Abhor that which is evil; cleave to that which is good.