13 “Then said Jesus unto them, Yet a little light is in you.” And by this it is you understand that Christ abides for ever. “Walk, then, while you have the light, lest darkness come upon you.” Walk, draw near, come to the full understanding that Christ shall both die and shall live for ever; that He shall shed His blood to redeem us, and ascend on high to carry His redeemed along with Him. But darkness will come upon you, if your belief in Christ's eternity is of such a kind as to refuse to admit in His case the humiliation of death.
“And he that walks in darkness knows not whither he goes.” So may he stumble on that stone of stumbling and rock of offense which the Lord Himself became to the blinded Jews: just as to those who believed, the stone which the builders despised was made the head of the corner. Hence, they thought Christ unworthy of their belief; because in their impiety they treated His dying with contempt, they ridiculed the idea of His being slain: and yet it was the very death of the grain of grain that was to lead to its own multiplication, and the lifting up of one who was drawing all things after Him.
“While you have the light,” He adds, “believe in the light, that you may be the children of light.” While you have possession of some truth that you have heard, believe in the truth, that you may be born again in the truth.
Source: Tractates on the Gospel of John (New Advent)