I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship.
(Romans 12.1 ESV)
Therefore I tell you, do not be anxious about your life, what you will eat or what you will drink, nor about your body, what you will put on. Is not life more than food, and the body more than clothing?…Therefore do not be anxious about tomorrow, for tomorrow will be anxious for itself. Sufficient for the day is its own trouble.
(Matthew 6.25-34 ESV)
At first glance these passages may not seem to be relevant to each other, but they are in fact inseparable for living the Christian life. In the first passage we are commanded to be living sacrifices; to offer all of ourselves in the service of Christ; to love God with all our heart, soul, mind and strength (Matthew 22.37; Mark 12.30; Luke 10.27). Jesus called this the “great and first commandment” (Matthew 22.38 ESV); “first” because it is the most important commandment and “great” because it is the furthest outside humanity’s ability to keep in our own strength.
The reason for humanity’s difficulty in keeping this commandment is that every human, even Christians, possess a sinful nature that is sometimes referred to as the “flesh” (See Romans 6-7; James 4.1-17; Galatians 5.16,17). Basically this nature opposes God and seeks either its own will or the will of Satan (this topic is deserving of further explanation, but such an explanation would make this post obscenely long). Since all possess a nature that inclines them towards fulfilling their own desires instead of God’s, and since humanity is naturally inclined to follow it, it is much more familiar and comfortable than following the Holy Spirit.
However if you are going to follow God you must begin to follow the Spirit and not satisfy the desires of the flesh. It is impossible to serve both God and the flesh (Matthew 6.24; Luke 16.13). So what happens when you start trying to live a way that is unfamiliar to you and you put your trust in someone that you cannot see? Well, you become anxious. Hence the teaching of Jesus in the second passage.
Jesus sees us as we really are, Fallen nature and all. He knows that when we begin to trust in him and follow the Spirit that we will become anxious because we have left our familiar and comfortable way of doing things. This is why he taught us to not be anxious. And because he knew that there was no way for us to leave our comfortable, sinful, and Fallen way of living life in our own strength he became human, died, and rose in order to pay our debt and make it possible for us to leave our selfish way of living behind and live for Christ instead.
These two passage are like peanut butter and jelly: they complement each other. Offer all of yourself for the service of Christ and don’t be anxious when you do it because you have left your familiar ways of doing things. You are more valuable than the birds and plants whom God takes care of, so he will certainly take care of you as well.