“That which” can refer to either the message or the person of Jesus Christ. The article ho (that which) could be neuter, which would suggest that John is referring to the gospel message instead of Christ. But by the descriptions that John employs, it need not be limited to a message. It could then very likely refer to Jesus Christ, who assumed flesh (John 1:1–18).1 The apostle goes on to speak about “hearing, seeing, and touching,” something that supports the view that he is referring to the person of Christ.
1 That which was from the beginning, which we have heard, which we have seen with our eyes, which we looked upon and have touched with our hands, concerning the word of life