The Greek relative pronoun hos/hē/ho introduces relative clauses and refers back to an antecedent noun. It is fully declined and agrees with its antecedent in gender and number, but takes its case from its function within the relative clause. Appearing about 1,405 times in the NT, it is fundamental to constructing complex theological statements. The neuter ho can introduce clauses summarizing entire realities.
Relative clauses introduced by hos often carry some of the NT's most concentrated Christological affirmations — what scholars call "Christ hymns." Colossians 1:15 begins a magnificent Christological passage: "He is [hos] the image of the invisible God." Philippians 2:6 (hos en morphē theou — "who, being in very nature God") and 1 Timothy 3:16 (hos ephanerōthē en sarki — "who was revealed in the flesh") use hos to launch creedal summaries. A tiny pronoun becomes the gateway to deep theology.