If they mean, 'I still pray mostly, read my bible and try to be kind to others, but don’t want anything to do with Sunday worship and religious denominations' then you could respond like this:
Well let's go and see what an authentic response to God looks like in the bible.
In the Old Testament if people were touched by the Spirit of God they were given a mission that would benefit God's people: Samuel, David, Daniel, Joseph, Esther, Moses, Solomon, Elijah, Gideon etc. So what's your mission, and how are you living it out?
In the New Testament when the Spirit of God touched them they were impelled to be baptized and join the Christian community, Acts 2:41-47. When God does something amazing in your life, just offering Him your own thanks and praise is not enough…you want thousands to join you in those prayers, and even that many would be hardly enough. So it might be time to question just how real your spiritual experiences have been, yes?
If they mean, 'All that really matters is me and Jesus. We're doing fine. He feels more real to me down the beach or walking through nature than in a stuffy church with stuffy people' then you could respond like this:
That's really weird. In the bible when people find Jesus He is rarely alone, usually there's Mary and Joseph, or 12 Apostles, or 72 Disciples, a synagogue full of people or a crowd. Why then do we hear God say so often, 'You shall be My people and I will be your God' and not 'You shall be my person'? Why does St Paul go on and on about us being members of the Body of Christ? Loving someone means loving and cherishing the people and things important to them. Revelations 21 talks of the holy city, the new Jerusalem being the bride of Christ, and Ephesians 5 says that the church is the bride of Christ. How can we possibly say that we love Jesus if we avoid getting to know and love His bride? As for 'doing fine', that's sort of hard to tell. Someone in intensive care looks like they are doing fine compared to a body in the morgue, but may not look so fine if they stand next to an Olympic athlete. So you need to go where the athletes-in-training are, and you will find them at various levels of fitness at Church. When we think of what Jesus did for us, forsaking heaven and taking on our human nature in order to suffer and die grievously to obtain pardon of our sins, 'doing fine' from our perspective might not be doing enough from the perspective of Jesus. Then again, the bible didn't fall out of the sky fully formed, and you would never have known about Jesus unless untold people passed the Good News down the centuries through many cultures and generations to get to you. Without the church you would not have Jesus. It sort of means that church not an optional extra, and that we need to be 'plugged in' so as to do our part to pass the Good News down to the next generations and cultures.
If they mean, 'I believe that human beings are more than flesh and blood, that there is a spiritual part of each person that needs to be nurtured and there are lots of ways to nurture our spiritual sides without having to believe in a supreme deity and without going to church' then you could respond like this:
Truth, beauty and goodness certainly nurture our spirits, and we need lots of experiences of them to remain healthy. But have you noticed, the human heart always wants more? There is a God shaped hole in our hearts that only He can fill, and we are – as St Augustine says – restless until He does. If you are more aware of spiritual things, then you are aware that there are benevolent spiritual influences and malevolent spiritual influences abroad in our world. Good is at war with evil. The forces on both sides are formidable. What kind of defense do you have against malevolent spiritual influences? There's more safety in numbers than for person alone.
Some of those conversations might have included: 'Could you please stop talking about God so much?' This question is actually an opportunity if you handle it right. Why? Because most people will respond with indifference, or just ignore or tune you out. If someone asks you to stop, then something you have been saying has hit a nerve – which means that it is highly probable that the Holy Spirit has been working on them. This means you can ask them gently, 'What have I been saying that has made people uncomfortable?' Which will give you an indication of where the hit nerve is: for example guilt at neglecting God, abortion, relationships outside of marriage. To which you can reply, 'I think you misunderstood me, or only heard part of the conversation: here's what I was trying to get across and the context of it….' Or 'What is it about …… (insert topic) that is painful for you?
It could, of course, just be a general complaint about a perception that because you always include something 'holy' with a gift or Christmas card or birthday wishes (eg scripture quotation, prayer card etc) that they think that you think that their lives aren't good and need fixing. Here's your chance to say, 'Well, that's not my intention. I am sorry you feel that way. I find these things helpful in my own life, and because I want the best for my friends and family I share them around. You are not being targeted. Everybody I care about gets them.'
So don't let these verbal objections - a.k.a. opportunities for meaningful conversation - pass by. Keep gently digging until the real reason surfaces, and then with the help of the Holy Spirit deal with it. It may need an apology, or an explanation, or a clarification, or a vocal prayer asking God to heal or assist whatever hurt or need has been uncovered. Most of all listen as best you can, and only give advice if it is asked for.
If your family and friends include people from other Christian traditions, then eventually conversation will swing to why they are so certain of getting to heaven, and why you should be too. The 'Once saved, always saved' is a very comforting notion, but it doesn't really stack up against scripture. But we live between the Now and the Not Yet, and Catholics express it this way: I am saved, I am being saved, and I hope to be saved. The biggest salvific event in the bible prior to the death and resurrection of Jesus, was God freeing His people Israel from slavery in Egypt and getting them to the promised land. On that journey each one of them had been saved from slavery in Egypt, and they travelled as a community to where God was leading them. If someone separated from the community, then they had no hope of getting to the promised land. Some of them died on the journey without reaching God's promises. Some of them forfeited the promised land by disobedience and lack of trust. But they could all truthfully claim that God saved them from slavery in Egypt - however to be fully saved they had to do more than stand still once they were outside Egypt's borders. Anyone on that journey of salvation could say, 'God has saved me from Egypt, God is saving me from Egypt, and I will be fully saved from Egypt when I reach the promised land.' Not only was remaining with the community essential, the manna God sent every day was essential too – and Catholics see in that an archetype of the Eucharist.
Salvation is a coin with two sides, we are saved from something and we are saved for something. The further along the journey we go, the more what we are being saved for matters.
If 'Once saved, always saved' is true, then 2 Peter 1:10-11 doesn't make any sense:
'Brothers, you have been called and chosen: work all the harder to justify it by good deeds. If you do all these things there is no danger that you will ever fall away. In this way you will be granted admittance into the eternal kingdom of our Lord and saviour Jesus Christ.'
If St Peter considers that there is a real danger of falling away, then there is a real danger of falling away and we have to engage our whole selves and our whole lives in God's service if we are to gain entry to His eternal kingdom. God saves us, and saves us mightily when we ask Him to become the Lord and Saviour of our lives, but He cannot give us the fullness of that salvation unless we do our part to co-operate with His grace that wants to make us holy and perfect and able to enter that kingdom. Nothing impure, unclean and imperfect can enter Heaven (cf Rev 21:27)
May the Holy Spirit always give us His answers when these conversations crop up in our lives.