1 Tim 6:15-16 is focusing on the resurrected and glorified Christ–the reasons for are listed below:

1. The immediate context deals with Christ. Paul writes about “the fine public declaration” that Christ made “before Pontius Pilate” (1 Tim 6:13 ). 1 Tim 6:14 also references the “manifestation of our Lord Jesus Christ.”

2. Additionally, the title “King of kings and Lord of lords” is applied to Jesus in the NT (Rev 17:14; 19:16). While similar titles are used of YHWH (Jehovah) in the OT/Tanakh, I’m not sure that the exact title, King of kings and Lord of lords is ever applied to the Father (YHWH).

3. 1 Tim 6:16 is evidently comparing the happy and only Potentate to those who rule as kings and lords. In contrast to these men, Christ is the “one alone having immortality.” However, God the Father is not the only immortal being, since Christ assumed immortality when resurrected by God; moreover, those who share in the first resurrection are also granted the gift of immortal life (Rom 6:9; 1 Cor 15:50-54; Heb 7:16).

4. According to Acts of the Apostles, Christ dwells in “unapproachable light” since his glorification. The apostle Paul beheld the glory of the resurrected Christ and he was blinded by the unique and awesome spectacle (Acts 26:12-13). Jesus assured his apostles that humans would behold him no more, but his disciples would, because he lives and they live through him (Jn 14:19). How appropriate the words of 1 Tim 6:16 describe the exalted Christ.