“Greet one another with a holy kiss. The churches of Christ greet you.” Romans 16:16 NKJV
Holy kiss depends on the cultural background. Some culture greets their brethren with a kiss to a cheek, some by kissing the hand, some by shaking hands, some by hugging, etc. It’s basically a Godly greeting meaning, whatever contact you used, greet your brethren in a Godly manner, genuine, sincere and without hidden agenda. Holy symbolizes Godly and without malice and, kiss symbolizes genuine, passionate and intimate. When you greet, show it without malice, Godly, genuine and passionate hence, THE HOLY KISS.
But then again, there are other figurative meanings of Holy Kiss that is sensible and connected to the verse below.
“Mercy and truth have met together; Righteousness and peace have kissed.” Psalms 85:10 NKJV
When you meet someone, it does not necessarily mean that you are automatically in agreement with each other.
Mercy, truth, righteousness, and peace are relative to the person who has it meaning, his/her notion or belief of the aforementioned may differ from another individual.
My personal truth and your personal truth may differ and to greet with a holy kiss could figuratively mean, we let my truth and your truth meet and whether we agree or disagree at the end, peace is still upon us.
Every Christian of the different denomination is claimant that he/she is a true church of Christ, the problem is because the claimants are of a different denomination, the teachings of what is truth and righteousness may differ in every denomination. The instruction was, churches of Christ greet one another with a holy kiss meaning, within the body of Christ, from one truth to another, let it meet. It’s kinda like debating or reasoning together about the Holy word of God with peace and/or shaking a hand or hug or kiss in the cheek in the beginning and at the end being done passionately and genuinely, no personal grudges and still friends even when there is a difference in beliefs.
Your truth, righteousness, and mercy meet with other truth, righteousness, and mercy with peace in the end. This can also be the greeting with a holy kiss figuratively.