The speaker compares the experience of confronting his own sins to someone who sees a dark grove but doesn't stop there—he looks past it toward the open sky. In the same way, when he sees his sins, he moves his gaze further back, all the way to the water of baptism. That water isn't ordinary; it springs from Christ's wounded side and rests above the heavens. It can either stop sins from spreading or wash them away with tears of repentance. Baptism becomes a personal, ongoing source of healing—spreading a "plaster" that perfectly fits every wrong, and teaching the book of life his name. Even if future sins try to accuse him, that first meeting with grace drowns out their charge. The mood is quietly confident, full of relief and trust in a cure that already covers everything.
H. Baptism — Summary
As he that sees a dark and shady grove, Stays not, but looks beyond it on the sky; So when I view my sins, mine eyes remove More backward still, and to that water fly, Which is above the heav'ns, whose spring and rest Is in my dear Redeemer's pierced side. O blessed streams! either ye do prevent And stop our sins from growing thick and wide, Or else give tears to drown them, as they grow. In you Redemption measures all my time, And spreads the plaster equal to the crime; You taught the book of life my name, that so What ever future sins should me miscall, Your first acquaintance might discredit all.