Atheists use this as derision, likening God to an imaginary friend; relating to God is like a child with an imaginary friend. Well is it? In some ways it is. Imaginary friends are said to be positive things by child psychologists. Through the assumption of God's active presence in our lives we can model holy living just as through imaginary friends children are modeling real friendships later for life. It really depends upon the extent to which people take it. I've never been comfortable with "Jesus is here invisibly" idea. I am not comfortable with the way of relating to God that assumes God is saving me a parking place. Some Christians sort of assume they are experiencing God and then letting God step into such occasions. That's actually not all bad really. The sense of God's presence, or what we call "God's presence" is documented over and over again in empirical studies as a valid life transforming experience and something that really changes people's lives for the better in dramatic ways. Some studies show that the mystical type experience is the most mature form of Christianity. The study by Robert Voyl shows this, and it links Christian experience to mystical experience.
That being the case we have no choice but to assume that the experience is the sensation of a reality that is actually present to us at the time. That's very different than the "Jesus is here invisibly" thing I refer to. That idea is about something the people who talk about it are not really experiencing. It's an assumption based upon the concept of God's omnipresence, and extrapolation from imagination. The actual presence of God felt by mystics and peak experiencers is a real palpable sense, it's not imagination.
The atheist can't evaluate this by just hearing about or reading about. I thought people who had such experiences were insane until I had one myself. It's as simple as this, you have to experience it. It's a way of life, it is not just one more hypothesis in a life of hypothesis testing. It's a relationship and develops over time. Not all Christians think they hear God, or even believe in that sort of interactive interpersonal relationship with God. There are many kinds of spirituality and many ways of relating to God. I went through my Charismatic phase in the 80s. I still believe some of what I picked up in that era, the "gifts" for example, miracles and healing. But I have ot tried to interact in that way, that God is my invisible friend, in some time. That is, in my opinion, a phase. It's the lower level of stages along the road to mystical union. Mystical union is the highest level of relationship with God and most Christians don't even know about it and will never get there. I will never get there in this life. But it is something, I believe, we will all experience in after life.
Mystical union is not in the Bible as such. There are verses that pertain to it, but it's not stated explicitly as such. It's part of the voluminous literature of Christian mysticism.
Diverse Expressions of Spirituality
There are as many different views of spirituality and styles of relating to God as there are people to do the relating. Christians are very different. G.K. Chesterton was as different from Billy Graham as was Adli Stevenson from Barry Goldwater. Which is to say as different as Clinton from Bush. Even within the closed ranks of Christian mystics it is very diverse. You don't have to relate to God like a big guy in the sky. You can relate to God like a principle or an idea. We can internalize the values and just learn to discern the will of God without having to "hear" or sense works or ideas. Atheists can't understand this because they have to assume it's all made up and so they can only go by words on paper. But they don't even bother to read anything except the bible and that they read for loopholes rather than understanding.
Atheists often confuse popular piety with Christian doctrine and spiritual experience. Popular piety is neither, it is not doctrine nor is it spirituality, at least not in a deep sense. The real depth in spirituality is the mystics; St. John of the Cross, Teresa of Avila, and mystical writers not saints, such as blessed John Riseborke, Madame Guyon, Baron Von Huggle. There are hundreds, or thousands. They are all different. They are as different from each other as Plato from Thomas Kuhn. Reading them will only give us a clue. You can't know God until you open your heart to him and begin a relationship with him. Until then it's only stuff you hear about and assumptions not in evidence. The first step is open your heart to God's love. Let God love you. If you think love is control and manipulation then you are just missing the boat on what life is all about.
Being a Christian is about knowing God in a personal way. This means experiencing God's presence, but it means a lot more than that too. It's a personalized relationship which fits the individual's own style; it's a love relationship:
Dear friends, let us love one another, for love comes from God. Everyone who loves has been born of God and knows God.
Whoever does not love does not know God, because God is love.
This is something that has to be experienced to be understood. Once it is experienced it is known. The atheistic claims to "just know" there's no God don't stack up to that, because they are by their very definition the absence of a relationship. One cannot experience the fact of a thing not existing. We can experience the lack of food, clothes, shelter, taxes, peace, whatever, but that doesn't prove these things don't exist, merely experiencing a want and a lack is not proof of anything. Experiencing the presence is proof of something. Atheists may assume or speculate as "what that really is experience of" but that is not the same as experiencing it. To have this kind of relationship with God is to know God. To know God is to love God because God is love. It is also knowing that God loves us. No one can understand this from outside the relationship and no one can judge God. People who think they are rejecting God are really just rejecting love.