"As," or more precisely, "just as He is pure": that is, Christ is pure from every sin and has no share in any sinful impurity.
Only the pure in heart can see God and receive illumination from the light of God's countenance (cf. Mt 5:8 and note).
Therefore purification of oneself is required of everyone.
Above, the Apostle teaches that the Blood of Christ purifies man (1 Jn 1:7). Here he says that man purifies himself.
There is no disagreement in this. Man purifies himself, or strives to purify himself, by the power and action of Christ's redeeming merits.
This is the same as saying that he is purified by the Blood of Christ, shed on Golgotha for the whole human race.
This Blood, or redeeming power of Christ's sufferings, is appropriated by man through faith in it, by the action of the grace of the All-Holy Spirit.
Compare 2 Cor 7:1; Eph 4:22; Col 3:9; 1 Pet 1:22 and the notes.