For with thee is the fountain of life: in thy light shall we see light.
For with thee is the fountain of life. No wonder that in thy house, and in thy presence, there are abundant streams, and a full 'river of pleasures' for thy people to 'drink of,' for "with thee is the fountain of life" itself, including the fullness of happiness, without which existence is death, not life (Psalms 16:11; Deuteronomy 30:20). No malice of foes can rob of life and blessedness these who have access to God, "the Fountain" of both (cf. Psalms 23:5).
In thy light ... "Light" is a figure for 'salvation.' (Psalms 27:1). Through the salvation which is wholly in thee (Psalms 35:3), we shall see salvation (Job 29:3).
Having depicted the malice of the wicked foe, and the fullness of life which the godly have in God, he grounds on the contrast the prayer following, that God would save His people and himself from the proud enemy, and confidently anticipates the downfall of the latter.