Obeying God is not Legalistic...
Obeying God through applying His commandments and directions in my life, not for the sake of salvation but because I am saved, is definitely not "legalism".
Simply put, obeying God is not legalistic, if we love Him.
It is also not legalistic to preach this truth to others.
Jesus said, “Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.  For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them , the same shall be called great in the kingdom of heaven.” Matthew 5:17-19
Jesus also said,
"If ye love me, keep my commandments." John 14:15
"He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him...If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him. He that loveth me not keepeth not my sayings: and the word which ye hear is not mine, but the Father's which sent me." John 14:21,23-24
"Herein is my Father glorified, that ye bear much fruit; so shall ye be my disciples. As the Father hath loved me, so have I loved you: continue ye in my love. If ye keep my commandments, ye shall abide in my love; even as I have kept my Father's commandments, and abide in his love. These things have I spoken unto you, that my joy might remain in you, and that your joy might be full. This is my commandment, That ye love one another, as I have loved you. Greater love hath no man than this, that a man lay down his life for his friends. Ye are my friends, if ye do whatsoever I command you." John 15:8-14
What are His commandments?
I am the Lord thy God, which have brought thee out of the land of Egypt, out of the house of bondage.  Thou shalt have no other gods before me.  Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth:  Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;  And shewing mercy unto thousands of them that love me, and keep my commandments.  Thou shalt not take the name of the Lord thy God in vain; for the Lord will not hold him guiltless that taketh his name in vain.  Remember the sabbath day, to keep it holy.  Six days shalt thou labour, and do all thy work:  But the seventh day is the sabbath of the Lord thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:  For in six days the Lord made heaven and earth, the sea, and all that in them is , and rested the seventh day: wherefore the Lord blessed the sabbath day, and hallowed it.  Honour thy father and thy mother: that thy days may be long upon the land which the Lord thy God giveth thee.  Thou shalt not kill.  Thou shalt not commit adultery.  Thou shalt not steal.  Thou shalt not bear false witness against thy neighbour.  Thou shalt not covet thy neighbour's house, thou shalt not covet thy neighbour's wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is thy neighbour's. Exodus 20:2-17 KJV
Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.  This is the first and great commandment.  And the second is like unto it, Thou shalt love thy neighbour as thyself.  On these two commandments hang all the law and the prophets. Matthew 22:37-40
A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another.  By this shall all men know that ye are my disciples, if ye have love one to another. John 13:34-35
In all reality, saying one must obey God is not legalistic, it actually is a requirement...
Read the rest of the Bible for more on this...