The quotation is from Psalm 95 and tells us that the heart may be hardened – literally become dry, dried out – so that you lose your assurance, your walk with the Lord, and your sense of his being with you. The word ‘harden’ referred to some foodstuff which, in becoming dry, became tough and inedible.
It can happen that you become loveless; you become tied once again to this vain world. ‘Heart’ here refers not just to the affections but to the whole soul, because this is quoted from the Old Testament; whenever you read ‘heart’ in the Old Testament, think ‘soul’. Sometimes in the New Testament, the word heart specifically refers to your affections, but in the Old Testament it always refers to your whole soul. So it means your affections, your feelings, but also your will, your determining factor, and your mind, your thoughts, and the operation of your conscience. Your grasp of truth in your whole soul may be dried out so that you don't think about spiritual things. Your conscience doesn't speak any more and warn you of sin and temptation.
Believers today are similarly exposed to all kinds of difficulties: those which all worldlings face, plus the additional trial of our faith, in that we are derided and scorned and rejected. The tragedy is that as believers, we can get awfully like ancient Israel: living for this world, triumphing in what we can do, what we can accomplish, what we can enjoy in this world. We can pray little, worship half-heartedly, come to church only once a week on Sunday morning, with scarcely any time or inclination for anything else related to God, not drawing from his wells for our pleasure and serving him. You haven't been to the Bible study or the prayer meeting when you could have gone, when you are not under pressure of duty or overwhelmed by some understandable thing. When you read ‘Not forsaking the assembling of ourselves together as the manner of some is’, you just shrug and say, ‘Well, it’s too difficult for me.’ You don't plan to serve the Lord and reflect upon him. When it is time for prayer or time to read the Scripture, that television is on and the attraction is too strong. The determining factor the will is no longer claiming your time for the Lord, committing you to carry out your spiritual duties. You say, ‘How far can I go with leaving out prayer, cutting it down, omitting my devotions or Scripture reading? If I don't feel so brilliant, skipping services, opting out of the prayer meeting. It is putting a strain on me and there is something else I'd like to do.’ How far can we go before God withdraws the blessing or something happens? Israel saw wonderful things over the notional period given here of forty years, so it was a wilful act. It is something we choose to do. I know it's not right for me as a believer, but I'm not going to dwell on it, as if I think about it too much my conscience will trouble me and I shall have to give it up. In all sorts of ways we toughen and harden the heart. So we become insensitive to these things and that's how backsliding dawns upon us. But here we are exhorted to consider: ‘This is a challenge to me. I must stop this practice of no longer attending, except when it's easy or except once on the Lord's Day, and I must review my program and honour the Lord and obey his word.’
Take some time to remember great answers to prayer. A week ago there was a very significant answer to prayer. Given the magnitude of that blessing, we should surely have given thanks for it for at least a week, and then yes it can be dislodged by others that take its place. Well you are in danger, because one of the problems with the people in the wilderness is that their hearts were hardened and they swerved aside and backslid because they were forgetful of the flow of wonders that they were shown constantly and day by day.
Respond to God's word and your spiritual duties with alacrity every day. Consider Christ, think much of him; that was the first piece of counsel in the passage. The second piece of counsel is, always act straight away in response to every challenge of Scripture. This is a great antidote to the soul drying out: the mind, affections, and will, no longer being on the Lord's side. Do we live the kind of Christian life we should when things go wrong? Do we ask the question, ‘Why might the Lord be allowing this present situation for me? What does he intend me to learn from it?’ We harden our hearts if you forget the blessings of God and give vent to moaning and complaining. Pull yourself up and say this is sent to me by the Lord to train me, to sanctify me.